http://collective.valve-erc.com/inde...source_mod_faq
seems to be the official faq for the upcoming HL2 SDK
Question #1:
Can I have the HL2 SDK now?
- Not yet. We are working on it now and will release it as soon as we think it is ready for public use. We understand that there is a great deal of anticipation from the community, and we want to put the necessary time and resources into the SDK so it is done right.
We are not distributing the SDK prior to its general public release.