I have written a converter for phpBB (prob an older version) to bbpress (the native Word Press version, Version 2.0-beta-3) for a client. Imports everything but the private messages at the moment. phpBB usernames had to be massaged to have spaces and punctuation removed.
This would not be free, but inexpensive, shareware prices. But a few beta testers to see if it might be generally useful would be nice. Contact me if interested.