The Pentax K200D and K20D are great cameras. Solid build, good image quality (in RAW; their JPEGs are not as good), and good prices make them my cameras of choice. Don't forget about glass though. Suggestion: Look for a K200D + 18-55 II + 50-200 for around $700. Great kit to get you started.
The Nikon D80 is a good camera, but is no better and actually inferior in many ways to the K200D (the Nikon has no built in stabilization, no weather sealing, and a higher price). The D90 has better image quality, but still has the same disadvantages as the D80, including an even higher price (that is certainly out of your budget if you include glass).
Canon's Rebel XSi is a nice camera with good image quality, slightly better than the K200D, but is smallish, plasticky, not sealed, and simply not as well built. Also, there aren't many old lenses that will work on Canon because of the FD to EF switch in the late eighties.
Sony has some nice options, but their lenses are rather overpriced and they don't have as strong as legacy in photography as Canon + Nikon + Pentax. Still, take a look at them and you may find something you like.
Olympus digital sucks because it's super-small format four/thirds and has NO compatible legacy lenses.
To pick, try holding the cameras in your hands, and see which one feels good. Pay some attention to price and features though.
