Can you not use a tripod?  If so a lot of P&S cams do pretty well with long
exposures.

You can't pull high ISO quality out of thin air.  You need large photosites
on the sensor, and that means either lower MP counts on a smaller sensor, or
higher pixel counts...but only on larger sensors.

You can't match the 5D, because it spreads 12 megapixels over a full
frame, and the pixels are almost the same pitch as a 6 megapixel model.
Your best bet would be to snag an EOS 350D before they disappear, or
even get a used EOS 300D. The 400D uses agressive NR to clean up a
pretty noisy basic 10 megapixel file, while the 350 is about as good as
you get in Canon for high ISO noise. I'd say some of the final
implementation Sony sensor 6 megapixel models are better at 1600/3200 -
the Pentax K100D, Nikon 50D in particular. But it seems pointless to
stray from Canon as a system, so the 350D/350XT makes most sense - with
a cheapie kit lens for lightness.

David

Leica M8.

Maybe some modest prime lenses would help, a 24 f2.8, 50 f1.8 (or 1.4,
or 60 f2,8 macro) and an 85 f1.8 (105mm if you have the 60) would all
be light weight lenses. If you like long telephoto lenses, maybe look
at Sony, Panasonic or Canon P&S superzooms, Though performance and
images on any ISO but the lowest will look noisy after the 5D.

Tom

It seems to me you're dreaming rather than asking for directions.  What
you have should surely be about the ultimate weight/quality/flexibility
combination.  Anything else will just be.... well, something else.
There's this, but it weighs over two pounds:
http://www.dpreview.com/reviews/sonydscr1/

Other than that alternative, the six to ten megapixel aps-c DSLR's
already suggested would seem to be the only (lesser) alternatives to
what you already have.  They will save you weight, but surely can't
equal the performance in any of your request/dream areas.  If you're
just looking for a solution and not a new system, then I would think a
zoom that fits your walk around needs mated with the lightest camera
should do the trick, regardless of brand since you're not talking about
a whole new system, but rather just a specific needs solution.

Having said that, I can't think there would be a better alternative
than just adding a wide to mid tele IS zoom to your lens collection and
mating it with one of the Rebels? (All around the same weight as the
prosumer superzoom with better everything.)  No, you won't have 13
megapixels to crop from or perfect low light performance, but to my
knowledge, that doesn't exist in the form you're wanting.  The only
other alternative that I can see is a medium format with a digital back
and hire a caddy to tote it around for you.  ;o)

To answer your question about reviews concerning signal-to-noise ratio,
be cautious about reviews.  Many use jpeg and even raw data that
has been heavily modified by the raw converters (and all raw converters
modify the data).  The smaller pixel size cameras are doing more
pixel averaging to hide noise.  I have sensor analyses at:

http://www.clarkvision.com/imagedetail/index.html#sensor_analysis

As a summary, see Figure 6 at:
http://www.clarkvision.com/imagedetail/does.pixel.size.matter

The "unity gain ISO" tells relatively how many photons each pixel really
collects.  You'll see that the 5D is at the top of the heap.
The other end of the intensity range, the read noise is very low
on Canon's CMOS DSLRS, 2 to 3 times lower than competitors CCDs.
So to go lighter than the 5D, the 20D/30D/350D would be the next
best choice for low light photography.  Among amateur astronomers,
the 20D/350D have been the most popular for low light astrophotography
with Canon, and the D50 for Nikon.

A 350D and a 50 mm f/1.8 lens would be very small, light, and
an amazing low light combination.

Roger

<snip>
Thank you all very much for the thoughtful replies. You've
been a big help.

Please consider this thread to be closed.
--David