SLR prices are deceptive because the kit lens that comes with them is of low quality. Quite literally, your camera is only as good as the lens on it. If you use the lens that comes with the kit, the camera will not perform up to its capabilities, making it pointless to buy an SLR in the first place. If you buy the camera body only, it will cost you roughly $400 for a good lens of the type you will definitely want if you buy an SLR. Pretty much, the only camera you should consider is the Canon Digital Rebel XT, which will cost you roughly $700 for the body only:
http://www.dcresource.com/reviews/canon/digital_rebel_xt-review/index.shtml
A better alternative for most people is to get a good fixed lens digital camera. For about $800, you could get the Sony DSC-R1, the best fixed lens digital camera on the market. This camera takes professional quality pictures, has a high quality lens and SLR grade electronics. It is equal to the XT with a $400 lens and superior to it with the kit lens. It's also is alot easier to take good pictures with than an SLR camera:
http://www.dcresource.com/reviews/sony/dsc_r1-review/index.shtml
If you are serious about the $600 limit on your budget, then your only option is a midrange, full size, fixed lens digital camera of the ultra-zoom variety. The best one is the Sony DSC-H5:
http://www.dcresource.com/reviews/sony/dsc_h5-review/
http://dcresource.pricegrabber.com/search_getprod.php/masterid=17744371
Probably, this is the camera you will be happiest with, for under $600. It takes VERY nice photos, albeit not of the professional quality of the R1 (like anyone would ever notice as a causal camera user, lol), has a high quality lens, superior digicam electronics (just not the high quality CMOS sensor you would see in an SLR) and the best screen on the back of the camera, on the market.