I'd personally love to see an HL2DM as the surprise. I won't give my hopes on it though. Although the rumors about it are enticing. But I'm sure TF2 would come at a later date anyways. I do remember reading that there would be an announcement about it soon after HL2 was released. But then again, maybe that's what the surprise is, just an announcement?
I don't know, too hard to tell. But I can't wait. SDK AND and surprise all in one week shall be very nice.
(And malex, I agree. I'd like to see a pre tag too.)