I'd say it depends on your backlog of games and genre interest. I had prepaid for an Xbox 360 during the summer, but as the launch lineup became clearer, I lost my enthusiasm for it (I'm sure by next Christmas there will be plenty of good games).
Long story short, I cancelled by pre-order and got a slimline PS2 with Katamari Damacy, Capcom Classics and Ratchet & Clank: UYA. This was the best videogaming decision I ever made - all three games rock and since then I've gotten Ratchet & Clank: GC and Dragon Quest VIII. PS2 seems to be the only system to have several AAA titles coming out in the first half of next year (Okami, Kingdom Hearts 2, Final Fantasy XII, etc). I say go for it now - there's just a ton of great games to play.