Leaving us already? I don’t think there is even an exporter for bbPress yet. If you can find a way to import into phpBB with RSS, you can just use the feeds from bbPress.
Trent
I’m looking for the same thing. I want to move to phpBB3 without having to manually re-write all of the topics. bbPress is brilliant, but there are so so many issues that just get a wee bit irritating. Does anyone know if there are any updates on this front?
The last I heard was that import/export was going to be a Google Summer of Code project, but I can’t find any updates there:
http://www.google.com/cse?cx=015986126177484454297%3Apfmwlvdl42y&cof=FORID%3A0&q=bbpress&sa=Search
Thanks! Haven’t found anything, but I’ll do a lot more searching, thanks for the link! 
I tagged this for Sam Bauers, the developer who posted the original information at the GSOC page. Maybe he will stop by and add some insight.
If you upgrade to the latest bbPress trunk you will have access to the new BBXF exporter. There is also a standalone BBXF importer for phpBB, but I have not tested it yet. You will have to do some work to get it running at this early stage of development, this should be a lot easier as we move forward with this idea.
For more check here http://bbxf.org/
