Nobody was bitching about Xbox user base when it only had 23 million sold. Why at 23 million sold does all of a sudden the PS3 have a small user base not worth developing for?
By that logic, why aren't they making Wii games. Or DS games?
Valve throws out xbox games because it's easy and they don't need extra staff duplicating efforts. They can develop their pc games like they always have, and toss an easy port to the 360 architecture. If the next nintendo or sony device has the same simplicity of porting, I have no doubt we'll see valve games on those platforms. If MS decides to go more proprietary, I have no doubt that valve won't make xbox games.
People get so caught up in fud about this because they're personally offended that valve won't develop for their favorite platform, it's all about easy. They've never said the ps3 (or wii...ps3 fans always ignore this) are pieces of shit.