Well, when Microsoft, the king of releasing software early, only gives out one beta, I don't see many being put out by Apple.
That's actually not true. M$ has put out betas since the 7000 build of Win 7, just none of them have been available to the public. You can find torrents of them if you want. The most published change in one of these privater betas is the ability to uninstall IE 8 if you want, something that will be in the official release of Win 7.