Brian's the only way to go. It's just how the name's always been spelt, since that Irish king. I think that's the source anyway. Why change it?
Bryan sounds like some ridiculous XTREME attempt to update it, developed in an Abercrombie focus group or something. No offense to any Bryans...
And yes, I've been called Brain for YEARS. I've even been called it in a newspaper article with my picture...
My guess as to why the mistake's made so often, beyond the fact that it's so easy to just switch the letter, is that it's easier to say than "Brian" and it's a word everyone knows so people slip.