I don't see it affecting 360 sales much as 360 only owners will simply get it on 360 - they won't buy a PS3 just for Portal 2. However, I do see the PS3 version getting a nice boost and that will be down to PC/PS3 owners. I believe there is a fair number of PC/PS3 owners who will get it on PS3 as this allows them to play it on PC anyway.
I've never bought a Valve game on anything but PC but I will get it for PS3 simply because it makes sense to have it for both rather than one and most PC/PS3 owners I know or see speaking on boards seem to be thinking the same way.
So I don't expect much change on 360 side but I'd be surprised if PS3 didn't sell better than it would if you didn't get it on PC essentially for free when buying for PS3.
Try to be reasonable... its easier than you think...
