I think the change of attitude among Valve and other developers is largely that they are learning that the PS-3 really does have more long-term potential especially as the console generation wears on, and the 360 begins to show it's age as it reaches the peak of it's capabilities. I look at the recent "Final Fantasy XIII" contreversy as a sign of that where it is pointed out that the PS-3 version of the game was superior, and superior enough where it's graphics were being used to sell the game for the 360.
What's more the PS-3 is apparently going to be the first to involve any serious MMORPGs, with DCU coming in a couple of months and apparently seeing a console release for real, and also the less exciting (but still noteworthy) arrival of "FreeRealms". Getting the "captive audience" of people playing MMORPGs on it is going to do a lot to promote the console. Especially if they manage to do "DCU" without the subscription fees as implied in some places.
That said I find it hard to get all that excited, and not just because I currently own every game machine out there. I respect Valve as a "shooter" developer, despite it not being my genere of choice, but they take so bloody long to build games, that it seems unlikely that we'll see anything new and exciting for the PS-3 for a while.
See, my inner cynic tells me that Gabe hasn't had a major change of heart with the PS-3. He's simply realizing that "Steamworks" will allow him to port games over to the PS-3 pretty easily, and that means Valve can take it's older library of games and "shovelware" them onto the PS-3 for some quick cash from a new audience. Basically re-selling the "Left 4 Dead" games for example, with maybe a slightly higher graphics resolution.
For there to be a real change of heart with Valve we'd need to see some PS-3 exclusives, or something truely new and innovative with a simultaneous release, and while that release might be "Portal 2" I still pretty much take an "I'll believe it when I see it attitude" because with their release schedule for all I know we'll REALLY see "Portal 2" bundled with "Half Life 3" which will be produced AFTER "Episode 3" whenever they finally get around to that.
Valve might make quality games (albiet shooters), but they are hardly a prolific producer.