If your using phpbb with Joomla, it may be a good idea to ask this question at
http://forum.joomla.org/. I really don't think the template system for Joomla is the same as themes for phpnuke so anything you ask here will likely not apply whatsoever to your Joomla theme.
I'd also suggest you get away from the Joomla wrapper as it makes it virtually impossible to link to your forum content properly. Anything used within the Joomla wrapper will assume the Joomla CMS url's I believe. If this is indeed the case, forget Search Engine Indexing of all your valuable forum posts.