(Jens Frank JeLuF@gmx.de): Hello,
A new user registered a few days ago using the name "Harry Potter". Warner Bros and JK Rowling have a trademark registered for this name.
If he writes a novel, or specifically portrays himself as somehow associated with the novels (other than as simply a fan), then there might be a problem. But trademarks are only valid for the particular purpose for which they are used: you can't trademark a name like "Apple", for example, and totally forbid anyone to use it for any reason. But Apple records can prevent you from selling records under that name, and Apple computer can prevent you from selling computers under that name, because they did it first and acquired name recognition in those specific markets. But if you want to call yourself "Apple" for your own personal reasons, that's fine.
Of course, none of that will prevent trademark owners from /claiming/ that you are infringing when even they know you're not, and making trouble. Until such a claim is made, I see no reason to be paranoid about it.