In a Facebook status update the developer revealed, "Today is very exciting day. We've been holding a big secret for the last two years and at GDC later today... We get to tell you all about it."
Sucker Punch has outright stated the announcement has nothing to do with Sly 4.
General chatter across the blogosphere seems to point to three possibilities: inFamous 2's getting some form of multiplayer mode, Sucker Punch has been acquired by Sony, or their next project will be multiplatform. All three evantualities seem plausible, though the timing seems wrong for a multiplatform announcement. Right now Sucker Punch is working closely with Sony to promote inFamous 2, a multiplatform bomb-shell would not be time appropriate. We also have doubts about inFamous 2 including multiplayer — if it was in the game it would have been announced a long time ago.
So that leaves some kind of acquisition. It would be nice to see Sucker Punch welcomed into the foray of first-party developers, but why would it be kept secret for two years?
As you can probably tell, we're pretty stumped. Feel free to speculate in the comments.