The Source 2007/Orange Box version of SDK Base, from which Source mods can run for cross-game compatibility, is available now. To use it simply change your mod's AppID to 218 in gameinfo.txt and restart Steam. If you have custom binaries, you will need them to have been built with the beta SDK code; access the beta through Steam > Settings > Beta Participation.
SDK Base Orange is only a 10MB download on top of the old SDK Base. Unfortunately it doesn't yet provide access to any shared Episode Two content, but that's most likely an oversight on Valve's part.