My policy is basically, have I got my money's worth with the systems I already have? Yes? Can I name a good dozen or so games I'd want on the platform I consider purchasing next? Yes? Then I'll consider buying. I definitely got my money's worth from the 360 and DS and out of Wii, PS3 and PSP, I'm most interested in the Wii so that'll be the next platform I get (please no Wii HD or Wii 2 anytime soon or else I'll scream). It just feels like it hasn't been long since I bought the 360 (along with the accessories and flood of games I got for it) and I want to uhh not spend so much for awhile longer. lol.
That all said, I could easily ride out this entire gen with the 360 and DS because it seems like games come out faster than you can play them. There are a lot of games I want to play that I haven't got around to playing yet.