>> I've been thinking about merging the non-toolkit tabs,
>> but this branch name is too restrictive.
>
> Well, branches are easy with bzr, so just create your own :-).

I've published a new branch that implements tabs for non-toolkit X builds.
It is at bzr.savannah.gnu.org/srv/bzr/emacs/x-tabs.
The tab bar is modelled after the tool bar.  Tab functions share
some similarities with frame-, window- and buffer-related functions
(same name conventions, etc.)