Thanks for the feedback.
User names in phpBB are indeed case sensitive, however, it won’t let you register an account with an existing user name, even if you choose a different ’casing’ (just tried it).
Might be for security reasons or to support individually ’designed’ nicks, but that is just a guess. Never really thought about that issue to be honest, as I always enter the proper case automatically.
Btw, which tag is Nokia’s? o: