Valve's confirmed that the surprise it has planned for E3 is "Portal 2 themed".
Confirmation of which came at the end of a note from the developer, which also confirmed it had delayed the game into 2011.
The note said:
"To ask questions about how close we all came to dying, or to ask futile questions about the previously announced E3 ***PORTAL-2-THEMED-FOR-GOD'S SAKE*** surprise..."
Let the speculation end. Once. And for all.