Kim Swift, the creator of Portal, will be on-hand at PAX Prime this weekend, unveiling her new game at the Square Enix panel on Saturday night. I would suggest you do everything within your power to make it to the show and check out whatever's on offer.
We have no word from Square as to what kind of game it is (or what it's even called), beyond this tantalizing reveal in a recent press release:
"PAX attendees will be the first to see Square Enix and Airtight Games globally reveal a top-secret joint project through their exclusive panel. Meet the development team, including Creative Director Kim Swift (PORTAL, Left 4 Dead 1, Left 4 Dead 2) and the rest of the Airtight Games team, and hear their explanation of how they are bringing an incredibly fascinating and quirky game to life. The panel will be held on Saturday, August 27 at 7:00 p.m. in the Pegasus Room."
If you're not going to be there, no worries. Check out G4TV.com for coverage of whatever Square and Swift are creating.