Canon Rebel (300D), Rebel XT (350D)
Canon EOS 10D, 20D
Nikon D100, D50, D70
All are good choices that can be found for around or under $250. All of them will perform better than your average $250 point and shoot camera in terms of image quality, especially in low light. All of these cameras are quite usable at ISO 800, some at 1600. While they are not as good as the most recent DSLRs, they are great ways to start off.
Forums (such as photography-on-the.net and fredmiranda) are good places to check for deals. I actually picked up a used (but excellent condition) 20D as a backup camera for $225 a few months ago, and the 20D is the most expensive of all of the cameras I listed above.