SDK has been released along with HL2's official Deathmatch mode.
http://www.steampowered.com/index.php?area=news
Shame they couldnt of had that done a couple weeks earlier, a lot of the games 9.5 scores woulda easily turned into 10's.
And even tho its only DM, this makes TDM and CTF mods REALLY easy to make now.