That's pretty much where I'm at, although my laptop isn't totally useless, so that is kind of an option as well. I plan to get a high end PC before long for 3D gaming, so most of my multiplats will probably go there for the 3D (unless I want to play with the DS3, Move or something else).

For me, it depends on the genre of the game:

RTS, WRPGs, Adventures (Point&Click), Shooters (rarely, since I don't really like Shooters) => PC

Platformers, JRPGs, Sport Games, Action Adventure, Fighters, Racing => PS3

Puzzle Games, Turn-based strategy  => both

I don't really know how I worked out which genre belongs to which platform, but that's how it "feels right" for me.

Dragon Age: Origins PC - Like to play Western RPG's with a mouse/keyboard

Elderscroll Oblivion PC - Mods, Mods and .....MODS

Red Dead Redemption PS3 - Most of my mates have it for PS3

Borderlands PS3 - Mates

Batman: Arkham Asylum PS3 - DLC and controls are IMO better on consoles for beat em ups.

Mass Effect 1&2 PC - Like shooters with mouse/keyboard and not worth getting 360 even though I love the series.

Then there's the PS3 exclusives which of course are on only one platform.

Mostly I prefer shooters/strategy on PC and Everything else on consoles.

I pretty much buy everything for the 360. In some cases I'll buy the PC version though, for example I bought Dragon Age: Origins on PC. I also get a lot of stuff via Steam as well (during sales). I was planning on getting ME2 on the PC, but since I had played ME1 on the 360, I stuck to the 360 for ME2 as well. Wii games I of course get for the Wii.

I haven't turned on my PS3 in a few months, let alone bought a game for it. The last time I turned it on was to watch a blu-ray movie. I was thinking about getting a new blu-ray movie though, so I'll turn it on again sometime in the next month.

I buy everything for my PS3,  for various reasons like Trophies, Friends and controller preference.  I have my 360 for the exclusives, and a few mulitplat games that are double buys.  Just recently got a gaming PC although I still prefer the controller over mouse and keyboard so this is really only for the exclusives.  

PS3 - for multiplat games / exclusives / friends / free online gaming 
Wii - for Nintendo games
PC - for games that are PC/360 exclusive thus canceling out my need or use of an Xbox360

PS3 = exlusives most multi-plat games (all HD ones)

Wii = exclusives a few multi-plat games

I get everything i can on PC cos of better graphics, customization and controls.

X360 for exclusives and console multiplatforms.

PS3 for Sony exclusives.

PC for nearly everything

PS3/360 for sports games and exclusives

Wii for exclusives