This is just the basics of running a profitable company.
While Valve probably should have tempered their words with regards to the PS3 platform, ultimately it doesn't make a difference to anyone other than to the PS3 fans (since general PS3 consumers aren't going to care so long as they can buy their games) who feel obligated to hold a grudge because a developer dared to slight and belittle their console.
Initially, the amount of resources Valve would have to devote to a dedicated PS3 development team outweighed what they measured as the potential benefits at the time.
After shipping 13m consoles in one year, with the majority of those coming from the post $299 MSRP, those are numbers that become harder to overlook.
Childish grudges aside, it's a simple matter of installed user base and potential customers vs. additional development costs/resources.
It's now becoming worthwhile for Valve to spare or hire additional resources for PS3 development based upon the way the wind is currently blowing. Reward outweighs risk.
The average video game consumer only has one current gen console. And while it may come as a surprise to some, many if not most of these same consumers don't have current 3D gaming PCs. Most are actually using laptops, and not expensive gaming laptops. So if that one gaming machine happens to be a PS3, it's by no fault of their own if they skip all of Valve's games for the simple lack of availability.
When Valve deems those numbers large enough to offset the costs of adding a PS3 R&D team, they'll do it.