Soon (hopefully that was vague enough). According to DHH - "Rails 3 beta is almost ready for public testing. We're just hammering out the last bugs in Bundler. Stay tuned." [1].
As far as I can tell from watching commits/tweets/etc, the finishing touches are being put on now. It sounds like there is still some work to be done on Bundler as well as finishing up javascript handling. Other than that, I believe that all of the big pieces are in place and ready for a beta.
Hopefully someone on the core team who reads HN can chime in with a better response.