The official word: is here:
Linden Lab is proud to announce that Project Bento is now part of the default viewer. Bento is probably the single most extensive update to the avatar skeleton ever, and as a result of countless hours of testing, updating, reconfiguring, testing... you get the picture, we are now live!
... Which brings up a very interesting question: Will Second Life's hardcore users, who overwhelmingly prefer third party viewers, switch to the official viewer so they can get all that great Bento goodness ASAP, such as amazing avatars like these, and incredibly articulated human avatars as this and also this?