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