"Cross Platform Play" is in all the titles I've seen, so I have to assume that there's going to be cross-platform play. Newell also talks about game invites, which are part of initiating a game with someone.
Portal should work well in cross-platform play since there's no direct competition: Quake III (IIRC) PC players absolutely dominated Dreamcast players. Collaboration removes the importance of this competitive edge.
Cross-platform play could also mean just PC-Mac. I'd just like to know if there aren't any 'gotchas', marketing folks are prone to these kinds of "soft lies" (remember BF2's 'dedicated servers'?)
“We designed the Portal 2 PS3 experience to be very straightforward for gamers,” said Josh Weier, project lead on Portal 2 at Valve. “PS3 gamers will be able to simply drop the Blu-Ray disc in the PS3, link to their Steam account from inside the game, and all their Steam friends (on PC and Mac) will be visible and accessible for chat and game invites.”
Portal should work well in cross-platform play since there's no direct competition: Quake III (IIRC) PC players absolutely dominated Dreamcast players. Collaboration removes the importance of this competitive edge.