I do not believe that usage of the GPL in the phpBB source code would put any requirements on licenses used in the templates, since they are entirely separate from the phpBB code. Also, since it is just css/js, I do not believe that there would be any requirements on license compatibility either, since they css/js can be considered independent since it is not technically required to display the page.
The only experience I've had with the Twitter Bootstrap has been a rather bad one, so I can't say I'd like to use it, but it would definitely be possible for third parties (or probably even phpBB itself) to make templates with the Twitter Bootstrap and release them.
My phpBB3 Mods
: Advertisement Management | User Blog Mod | Anti-Spam ACP | Advanced Subscriptions | One Click Ban | From Author PM List | FAQ Manager | Forum Sponsors | Soft Delete | Auto Database Backup | Drag 'n Drop Forum List | HTML Ranks | Enable HTML