According to this new message posted by Scott MacVicar of Facebook, the first versions of HipHop have officially been pushed to their account on github.

Today we've pushed an initial version of HipHop onto GitHub at http://github.com/facebook/hiphop-php. This source should build on CentOS 5.2 and Ubuntu 9.10. Any specific hard to find libraries are bundled where the license permits. Remember that while we're running HipHop in production it might not yet be stable for you. You should be comfortable with this sort of software as it's the first public release.

You can read more about the release on its github wiki and how to get it installed. Issues can be reported to this list.