Not many do use joomla, it's been in 1.5beta forever
Now that it's golden it might be a good time to look at it again. There was an integration done for mambo/joomla a few years back for the threads 6.5 series, but that won't work at all now.
I remember seeing an integration (I think it's called jfusion or something) that integrates other forums, might be easier to add-on to that one.
