The game is case sensitive (as is the register script when making an account), however the forums are not. This is a HUGE issue, as the forum throws up a duplicate error when this happens, essentially I could go around and create a forum name like "dAtOneGuy" and block myself out of my account, this can happen to anyone, and would require making a second account just to report it here.
I had 2 alternate accounts and noticed this a while ago, then realized how this could be a big issue down the road if someone decided to attack someone on the forums, pretty simple way of doing so.
I wish I had a proposed solution but any of them seem bad from a technical standpoint:
Contacting owners of all accounts in question and coming up with a solution for each (Is nice because it means no nasty merged accounts or dupes)
Mod phpBB (Don't remember if there's a mod for it) to be case sensitive (Seems the swiftest)