There is a story up at Wired about Spore [flash heavy site]. Although spore is a single player god type sim game it will have some kind of internet access facility along the lines of the sims, you can place your planet into an online universe. Whether your creatures can then interact with creatures created by other players is unclear but the idea does sound like lots of fun however it works.
Spore looks to be released sometime in 2006, presumably for PC. Hopefully a Mac version will come out at some point as well.