Get in there before the ceremony, day before or whatever and do some
practicing and adjust as needed.

One thing to watch for (I have shot myself in the foot before
with this one) is what lights do they have in the room.
The only thing worse than having to color correct your
pix for the room lights is to have pix that are partially
correct for color balance from your flash and the other
parts are not completely illuminated by the strobe have
the color cast of the room lights.  Almost impossible to
fix that one.  Yech !!

As others have suggested, take someone dressed in similar
colors to the room before and experiment to see how your
system works and how the colors look.  I would also
consider bumping the ISO up on the camera - as someone
else said the other day, "I would rather have sharp pix with
a little noise than no noise and blurry from motion etc".

Spare batteries for both the camera and flash.

You might want to wander around Ken Rockwell's site
and see if any of his articles help -- he has a bunch of
good stuff there.
http://www.kenrockwell.com/

Hope some of this proves useful to your task.  Good luck.

mikey

Use the fastest aperture lens and bounce your flash off the ceiling
wherever possible. Alternatively get some white cardboard, cut out a
paddle shape and use a rubber band to attach it to your flash. Bounce
the light out of that.

With Canon, use your FEL (*) button, but don't rely on it too much.

Shoot in RAW.

Don't stand too far away from the subjects. Get as close as you can
without invading their personal space.

Carry lots of spare AA batteries for your flash.

Is it on a military facility?  If so, make sure you have permission to
take pictures,
in writing,
in triplicate,
in blood.

In addition to the advice already given:

- be careful with exposure - white uniforms may easily overload the
highlights so setting the auto exposure down 1/3 or 2/3 stops may be an
idea.  Try to check in advance.

- don't fiddle with changing lenses all the time.

- get someone to take pictures with the P&S as well, just in case.

David

Get access to the room ahead of time and do a pre-shoot--a "shoot before the
shoot".  Shoot some test shots of someone in the uniform that will be
worn--from what you're saying it sounds like service dress blue, which is
mostly black, so contrasts will be an issue--run through a range of
exposures and examine the results later to see what seems optimal.  Make
sure that your test subject goes through some typical movements--"for
photographic test purposes only, hand salute" is likely to be the fastest
movement unless someone trips, make sure you can get that (the salute, not
the trip) without motion blur.  The 28-135 would be my first choice shooting
handheld because of the image stabilization, but make _sure_ that it is fast
enough to stop motion blur--if not the 28-75 2.8 would be the right choice
among the lenses you listed.  If you're not shooting RAW then check your
white balance beforehand and make sure it's coming out right in the
prints--ideally use the "custom" setting with a white uniform shirt as the
reference.  If you've not used custom white balance before then practice
setting it a few times until you're comfortable doing it, and once you are
wait a few days if you have the time to do so and practice it again (it's
surprising how much the second practice helps retention of skills).

If you're at all unsure of the exposure at any point then set up autobracket
and bracket the exposures--if you've not done that before you should also
practice that, including shooting a few shots just before the ceremony
starts--with a Canon DSLR you have to hold the shutter for the three
bracketing shots and it takes a little practice to always hit three and
never two or four and if you're like me if you haven't done it in a while it
will take a few shots for the timing to come back.

In a large room, unless you've got enough flash to illuminate the whole
place try to avoid it if you can, and if you do have to use it, as someone
else pointed out, make sure that it doesn't clash with the white balance of
the room lighting.  These are things to test in your pre-shoot.

Make sure your battery is fully charged and make sure you have enough
storage (and make sure you have it on your person and know _where_ on your
person--you don't want to have to rummage in your bag or turn out your
pockets for it).  Filling up the CF in the middle of a shoot and not having
another can ruin your whole day.

If possible, dry run the whole ceremony (you don't need the actual people
who will be there, just someone who knows what to expect and some stand-ins)
so that you have an idea what is coming and aren't out of position for
critical shots.

Someone suggested shooting RAW--if you're used to shooting RAW and have the
tools and skills to work with it then this is sound advice, if you're not
and you don't then something that is a once-in-a-lifetime event and
important to the participants is not IMO the place to learn.

Find out what other photographers if any will be there (for example will
there be a stringer from the newspaper or a TV crew?) and make sure you
don't all trip over each other.

You'll want shots from the waist up, mostly, for your friend and some
wider shots for the setting.  That means telephoto and normal-wide.
Probably the 28-135mm would be a good choice.

That doesn't actually matter.  With fixed lighting you can set the
camera to manual exposure and not worry about metering.  Try a couple
of shots early on to set the exposure using the histogram display and
then leave it set until you go outdoors.

Of course, if the room has dark spots then you'll have a harder time
of it.  You can try using RAW to get a little more exposure latitude.

Forget the flash if you can.  It doesn't allow bounce and I dislike
the flash look.  Of course, a flash look is better than no shot at
all.

Too bad it's slow.

That would be good for the closer shots as it's a faster lens.

Don't bother.

To me this situation requires:

1) Get in there and chimp your way to the correct flash and aperture
setting.
2) Set that aperture manually (f/5.6 to f/8) and leave it alone.
3) Leave the flash in TTL metering (but you may need flash comp).

In these settings as you change composition, the metering will reflect
the one drawback of TTL metering: it sets according to what it's pointed
at for each image.  In a "set" situation letting the AE set differnet
exposures for each frame results in a very inconsistent look across the
set of images.

With a lot of white uniforms, the camera will tend towards underexposure
by as much as two stops.  So, don't be surprised if you need +1.7 to +2
exp comp to get the exposure right.  It is hard to evaluate blown
highlights (white uniform areas devoid any of texture) using the camera
monitor.  Zoom in and pay attention to where the camera indicates blown
highlights.  (A few spots of blown highlights are okay).

Why manual?  Becasue once you do find that right setting, the next goal
is consistency across the set of images.  If you set that aperture right
and leave the shutter speed alone you will get consistency in exposure
that will make editing and selecting much, much, easier.

Since your flash is relatively low GN, shoot at ISO 400 to give your
flash a chance.  If the ceiling is white and at a height that allows you
to bounce off of it, that will soften the light on subject and eliminate
harsh, hard shaddows in the BG.  At the distance you mention, however,
this might not be possible.  Borrow or rent a higher power flash for the
bounce might be a way.

If you have enough memory, shoot RAW.  This will give you the best
dynamic range for possible corrections post event.

As for autofocus, be carefull if there is not a lot of contrast in the
focus sensor and if the light level is low.  Consider manual focus if
the chimping reveals a lot of missed focus shots.

Cheers,
Alan