What we have known so far...
> TF2's getting a Pyro Update
> The release of the Source Filmmaker - Make movies and videos with your gameplay with all Source-based engine games like TF2, HL2, L4D2.. and so on.
> MEET THE PYRO - DAY 3
>New Map and New Weapons for the Scout, Soldier, Sniper and of course, Pyro. - DAY 2
> A new gamemode: DOOMSDAY - DAY 1

From the Doomsday Game Mode Page Wrote:Doomsday pits BLU against RED, with only one team getting the honor of transporting a suitcase nuke full of recalled Mann Co. Australium to American monkeynaut Poopy Joe’s rocket, so he can blast off and hunt down Soviet space chimp Vladimir Bananas. It's the same spirit of competitive enterprise that stalled the moon landing by three years when Buzz Aldrin suplexed Neil Armstrong into a pile of folding chairs at Astromania '69! Both teams will attempt to get the necessary fuel up a preposterously slow-moving elevator and into Poopy Joe’s Australium-powered rocket to the stars.