Valve just announced that Portal 2, with Steamworks, will also be coming to PS3. That's the massively conjectured "surprise" they've been teasing for the last few weeks. On the plus side, we did get to see an awesome demonstration of Portal 2 in action, and we'll have our impressions of that up shortly. In the meantime, here's why this surprise suddenly makes sense - plus a brief Portal 2 trailer from the conference.
We did warn that Valve's surprise probably wouldn't be the megaton everyone seemed to be expecting. The 'surprise' was intended to replace a suddenly cancelled pre-E3 event at the Reagal Theatre in LA. In the end, Valve made their announcement as part of Sony's E3 press conference. It seems likely that after that event was arranged, Sony invited them to make the reveal as part of their main press conference instead. Obviously you can't pass up an audience like that, so they aborted their planned event and made a fun little joke out of it.
Really, the only problem with the whole thing is that Valve have so many exciting projects that announcing a 'surprise' tends to make our naive imaginations run wild.
Here's the Portal 2 trailer they showed: