I think Valve's third surprise for 2011 won't actually come as much of a surprise. Maybe it will be the announcement of an upgrade to their Source Engine-- I'm pretty sure they've been working on it a while.
I'm also pretty sure they realize that fans expect "Half Life 3" to be the best game EVER, and the current Source Engine is starting to show its age, so I think that upgrading it would be the next natural step before any new Half-Life related release. That's what I'd do anyway...
 
PS: yes, I know I wrote Half-Life 3. I don't actually think there will be an "episode three" at this point, but an actual third game.
 

edit: sorry to those who had already commented on this... I wanted to edit it and pressed the delete button, lol. That's what you get for writing blog entries while being at work :P