I honestly think the BEST DSLR under $600 right now is the Sony A200. Yes I could be a little biased since I own one, but it comes with a pretty nice kit lens, Built in the camera Image stabilization (sony calls it "super steady shot") and the ability to use any Minolta AF lens made since 1985. There is also the Canon Rebel XT, which is a good camera with a great following. Nikon makes nicer lower end DSLR's, but if you wanted to use an older lens, the low end nikons dont have a built in autofocus motor, and most older lenses would be manual focus only, canons all have the motor in the lens, and sony incorporates the motor into the camera.
The only thing I wish sony would implement on the A200 is a mirror lock up feature on the 2 second timer. While it isnt a big issue it helps make landscape and sometimes macro shots even better when using slower shutter speeds (even on a tripod). Maybe somone will release a firmware update for it...*dreams*
theres also olympus and pentax, but I dont know much at all about them.