Suppose you are running a Django site, and have a legacy PHP forum to support and integrate into your site, since current Django forum solutions are not mature enough.
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When I read the question summary I immediately thought that you would need some kind of script, which could be linked to a signal via the Dispatcher in Django, to syncronize the user database from your Django site to the forum. This would keep the authentication side of things in check - but you still need to do one of the things that Oli has suggested, to make them look the same.
Themeing will probably be the least hassle-free route, but that's not to say it will be easy!