In a succinct e-mail, Gabe Newell has confirmed that the "surprise" is not Half-Life 2: Episode 3. So, there it is, Internet. Wrong again. You're just going to have to go back to your second love of pornography to mend your bruised ego.
THIS IS A GOOD THING. That means the surprise will be good, as opposed to exactly what all of us are expecting. If we want to be insanely speculative, Valve did hire the people who made DoTA a while ago...... VALVEHON! WITH HATS!
Or was is Blizzard who hired them.... someone confirm this for me, pretty sure it's valve...