I have seen noise discussed quite a bit when i was researching my next purchase, now the 50D. Didn't really experience the issue until I took a photo at a horse event at iso 200 and later seen quite a lot of noise on the face heavily shade by her hat. Mind I have only just got the 50D and still in learning mode. I have to admit that my 400D was much more forgiving. I have taken a number of other shots at lower and higher iso and definately is not as bad. Did some looking, again; on the web and came across a thread that turning off the Auto Light Optimiser (ALO) feature has a big impact on reducing the noise in shadow issue.
When I did a search on ALO there was a bit of a following to the same. Hence I have turned off the ALO feature but yet to test.
Luv the 50D and to date this is the only one thing I can fault with it but in favour of the camera, I am still to learn correct usage coming from my 400d.
Interested to hear thoughts comments from other 50D owners.