CNet article on the differences between the forthcoming PS3 Slim and the old PS3 "Phat". Short version of the article: not much, the Slim is, well, slimmer. And it doesn't have the backwards compatibility for PS2 games that the Phat does.
Many people who paid for a PS3 "Phat" are now pissed that both the Slim and the Phat are gonna cost $299. If you already have a PS3, though, have a little forsight...don't buy the PS3 Slim. I imagine it's only a short matter of time before they come out with the PS4 to get another $500 from you.
Disclaimer: I know nothing about PS3, slim or otherwise.