This morning I've been working on a punbb plugin to insert the necessary Google Analytics code into the footer of a punbb forum. I got it working, but figured it might help to ask the punbb forums for advice.
When I visited punbb.org, I was greeted with a big message about how the creator has sold his copyright to the software. Congratulations to him! That's awesome. As explained in the message, it doesn't mean much for the existing project, as its been historically and is currently released under the GPL. However, you never know what the future holds.
Given I've already made some minor but significant alterations to punbb (mostly to power multiple forums on separate domains from the same codebase), I may end up forking the code and converting it to the Affero GPL, if that can be done.
Either way, I'll be releasing the Google Analytics Code plugin soon. Right now its very rough in that you need to enter your code, then go to the options page and save to have your code entered into the cache. Also, I'm not sure how to register the plugin to be called at the footer, so I put the code for that in the actual footer, which isn't a good idea I'm sure.