I'll call the next release 1.0.beta1.
I don't think there's a need yet for a new branch, since most of the changes in Dovecot core are bugfixes, and the rest are simple enough changes that they can't really break. And I'm not planning on adding any new larger features to Dovecot core either.
dbox format lives just fine in its own little world. If it's buggy, it doesn't really matter as it doesn't affect mbox/maildir at all.
The added plugins also can't break things since they are plugins and aren't used by default. Of course, there's that one compilation problem that I'll still need to fix..
As for when 1.0 comes, well, there are still too many unsolved bugs. It's good that people report backtraces from crashes here, but too often I just can't figure out how to reproduce them and can't see anything wrong with the code either. It would really help if people tried reproducing the bugs themselves and told me how it can be done..