Personally I think while they aren't all appealing to the same demographics the same amount they are obviously in the same market. Someone that buys a wii is less likely to also buy a 360/ps3.
I don't buy this split market dichotomy that people always bring up in relation to the Wii. There's no doubt that Nintendo's Blue Ocean strategy is working, but to conclude that some who buys a Wii is less likely to also by a PS360 is a bit of a stretch. A lot of people who bought the Wii are hardcore gamers, that's undeniable. Given Wii's lack of HD support, horsepower, and exclusive games like GTAIV, etc. I'd say many Wii owners are likely to go PS360 to complement their Wii (and vise-versa if you want Zelda, Mario Galaxy, Smash, etc).