There's a few things that concern me about this. The first being compatibility issues, the only other game that I can think of that did console and computer cross play was Shadowrun for the 360 and PC. While it used the same service (Xbox LIVE) PC gamers had to pay the fee in order to play online period, which is something they all had issues with. Given that PSN is the usual service and Steam pretty much takes care of itself I sense issues. The article does say that the PS3 version will have Steamworks included, but the question is how well will it work?
There's also the issue of input controls. While I genuinely prefer a game pad over a mouse and keyboard as I understand it PC users generally have an advantage and can react faster thanks to quicker and smoother controls. This may make for frustrating experiences and end up with PC users only looking for PC users to cut out the issue.
And of course there's the issue of whether or not Microsoft will get in on this and I'm making a prediction of "No." Simply because Microsoft will in no way shape or form interact with Sony and because of that Valve will just design the game to use Xbox Live instead of Steamworks. Making it so only 360 users will play with 360 users. (Which, after rereading this to myself probably won't be a bad thing for me since I only have a 360 and tried Steam and disliked it.)
There's also the issue of Gabe Newell being a hypocrite (read distaste for console platforms) but that's answered simply. Sony gave him money, he becomes their bitch.
I'm sure Portal 2 will be a great game, but this cross platform thing doesn't strike me. It's probably going to end up being ignored. And if that's the case why bother with it?