Well, I bought a PS3 first then a Wii, so unless I'm the only person in the world to do so I would imagine some other HD console owners will do so too.
It's down to what they want, of course. The Wii complements my PS3, but I wouldn't be looking to play FPS or games where I believe HD graphics add to the experience, so I'm sure many PS3/360 owners simply won't see any appeal in the Wii's games/experience (which is fair enough).
I do suspect its more likely PS3 owners would also get a Wii than 360, but I could be wrong. Most 360 owners I know are passionate about online shooters, Live and the 360 in general, and show very little interest in Wii (and in fact tend to bemoan its existence). Most PS3 owners I know who don't have a Wii speak about getting one at some point.
Of course that's a small sample, but its the only one I know!