IGN is reporting that a trusted source at GDC "overheard" the news that Insomniac Games, a reliable Sony partner for over a decade, has gone multi-platform, and is currently developing a game for both the Xbox 360 and PS3. The rumor is being reported by Matt "Megaton" Casamassina at IGN, so take it with as many grains of salt as you need, but if true, this would be a pretty radical shift for the company and possibly cause some holes in Sony's exclusives schedule.
Insomniac is known for the Ratchet and Clank and Resistance series of games, and is currently believed to be working on the third game in the Resistance series, due either late this year or early next. They recently opened up a second studio in North Carolina, and while rumors of a new IP have floated around for a while, nothing solid ever surfaced. If this rumor pans out, it's likely this new, unannounced game would be the one they're developing for multiple systems.
As an independent company, Insomniac are free to develop games for any console or the PC, despite their previous system loyalty. Pretty much they'd only have to deal with the inevitable cries of betrayal and rending of garments from Sony fans. Despite the PS3's improved fortunes of late, I imagine it's still pretty tough out there for any Sony exclusive developer not named Naughty Dog. While critics generally loved the last Ratchet and Clank game, players (and the developer) might be feeling a little fatigued after 5 games in the series, not including spin-offs. Originally touted as a Halo-killer, Resistance hasn't quite lived up to that potential; in particular, Resistance 2 was considered a disappointment by many players and critics alike. I wouldn't blame any company for doing what they need to stay afloat these days, particularly one with as much raw talent as Insomniac has demonstrated. So far the companies aren't commenting (except to say, "No comment,") so we'll just have to wait for now.