The Joomla! community just completed migrating their forum from SMF over to phpBB3. Brad Baker posted some of the details on Joomla.org's use of phpBB3 for their forum. In part, some of the move to phpBB stemmed from Joomla.org's discomfort from bridging GPL applications with non-GPL applications. Baker answers the question in his own way.
Why did we move to phpBB3?
Good question, and there is a simple answer. Whilst SMF was great for us, it is not licensed under GPL, and as a result many of our users who like to follow the choices we make were going to run in to integration issues in the future. As well as that, the team at phpBB have been great, with many of them offering to help, especially with the conversion.
In any case, we’re not going back, and the future is phpBB3!
Beyond a few minor tweaks, it looks as if Joomla.org was able to convert SMF over to phpBB with very few issues. Considering the Joomla community forum usually has hundreds of users on line at the same time, this is no small move for a forum to make the switch to new software. Congratulations to Joomla!