larmir,
I had a similar thing happen to me on WordPress. Just about went nuts trying to figure it out, but I finally did. I would think that since bbpress is made by the same folks who did WordPress that you’re experiencing the same thing I did with WordPress. I’m guessing that when you created your database that you didn’t create it in UTF-8 format. If you check the installation documentation, see if it recommends UTF-8. Then, check to see what kind of encoding is on your database. If that is the problem, you can solve the problem fairly easily. Here’s a link to instructions that I wrote for fixing the problem in WordPress:
http://www.minc.info/?p=32
You should be able to do the same thing to fix it with bbpress. *NOTE* I haven’t tried this with bbpress so I can’t guarantee it will work.
Hope that helps,
Ben