I use a 24-70 F2.8L USM - it's very versatile, and produces very
high-quality images on a 20D

You want a flat, slightly telephoto lens with a widish maximum aperture: the
flattening effect tends to be more flattering, these lenses are sharper and
have less distortion.
All rules have exceptions if you know what you want and how to achieve it.
Generally 35mm/dSLR portraiture works best with a slightly longer than 50mm
focal length, usually in the 85-100mm category even on an APS sized dSLR
sensor. You must experiment and see what works for you.
I am sure Canon users have many personal preferences. I am not a Canon fan
and express no specific recommendation.
You most assuredly do not want a zoom lens for critical portraiture.
IMHOP lighting and the mood you create for your subject are actually more
important than the camera and lens.
Also there are wonderful ways of modifying portraits with Photoshop that are
very pleasing to subjects that probably also are more important than
specifics of camera and lens.

50/1.8 doesn't have the reputation of having good bokeh. Most people
suggest 50/1.4 or 85/1.8.

The 85mm f1.8 would be a better choice.  Much better bokeh than 50 1.8
which has a pentagonal diaphragm.

Probably a bit too sharp, but the perspective will at least be right.

Today fatboybrando commented courteously on the subject at
hand

If you mean portaits of people, you might look to something
longer. Pros often (but not always) use from 85 to 105mm lenses
to do facial portaits to intentional foreshorten human features.

People tend to like the look because it minimizes "defects" they
don't like in themselves such as large noses, large ears,
receding chins or hairlines, large Adams Apples, and other
things that look a little more flattering when flattened some.
The opposite is also true - taking portraits with a 35mm lens
will make most people look like Alfred E. Neumann and the
photographer won't get much business once the "word" gets
around.

It'll be tough to get a lens that will serve as a portrait lens on both a
1.6x crop digital camera and a 35mm film camera.  A 50mm figures out to
80mm, which is within the realm for a slight telephoto for portraits.  Most
prefer a length between 70 and 100mm, sometimes as long as 135mm. So, a 50mm
will probably be too short for the 5, since you'll have to get very close to
your subject to fill the frame, and that could lead to some distortion.  But
an 85 f1.8 or 100 f2, my personal preference on 35mm are too long, equaling
a 136mm (at the long end of useful) and 160mm, only useful if you are
shooting in a barn...

Sean

I was using the 50mm 1.8 yesterday evening for portraits of people and dogs.
I had started out with the 24 - 105 L F4 but changed to the 1.8 to see how I
could better manipulate the DOF.  I was very happy with the 50mm.  The bokeh
seemed fine to me but maybe I am not to critical.  Was shooting in the late
evening and the colour and contrast of the 50mm was great.  For the money on
a 350D its pretty good value (I was using it on a 1Dmk2n).

regards

Don

Wait a few days, and I'll post a picture with the results of the 50 1.8 II.
I just ordered one from B&H. I've been meaning to for a while, but after
reading how it's just too good a deal to pass on, I ordered today.

Look here:
http://www.pbase.com/jeremygood/50mm_18

I think the shots speak for themselves.