I've been true to Nintendo for a long time (NES, N64, Gamecube). But I decided that when I get a console (likely in the spring) it'll be a 360.
Between the graphics, online capabilities, controller ( 👇 to the Wii controller), games, my brother and friend have one, etc. I can't go with the Wii. I already played Twilight Princess. The only Wii-specific game I'll miss is Smash Bros. and maybe Metroid 3. The upcoming Marvel Universe Online was the kicker for me...only 360 and PC, and scattered others that I'd rather have with a normal controller and better graphics.
3rd-party publishers seem to cringe at Nintendo these days, and the PS3 is just too expensive and without anything tantalizing.