I'm having the same problem. I just installed punBB over the weekend and I haven't made any changes to any db tables, but when I try to register a new user I get:
File: /howto/forum/register.php
Line: 135
PunBB reported: Unable to fetch user info
Database reported: Illegal mix of collations (armscii8_general_ci,IMPLICIT) and (latin1_swedish_ci,COERCIBLE) for operation '=' (Errno: 1267)
All other forum functions seem to work fine. Strangely, I was able to register a user right after I installed the BB, but now a few days later I can't. All tables have collation armscii8_general_ci and none have been changed since installation. All my other (non-forum) databases are all armscii8_bin, but that shouldn't make any difference to the forum database.
I get the same error with Foxfire and IE7 (which is not surprising considering it's a mySQL error)
Any suggestions?
--gary