[font color=\'#000000\']I've been curious about this question for a while. I've been using a D60 for the past two years almost exclusively for night photography. I can give you an answer based on my experience with the D60, and from some postings on DPReview about the 10D.
The D60 maxes out after about 4 minutes. At that time you may get a few hot pixels, but they can easily be removed in PhotoShop. At 4-1/2 minutes, the image becomes unrecoverable.
From what I've heard of the 10D, some users say they start to see hot pixels after five minutes. Others claim they go over 10 minutes without any hot pixels. One user told me he got 18 minutes with no hot pixels, but when I looked at this image, they were definitely there, although it was comparable to my D60's five-minute exposures.
And the 20D is whole new bag of worms. I haven't heard any long-exposure results with the 20D, yet. If you hear of any, please let me know.[/font]