I guess I don't really need an explanation on mine...next time you're playing any classic game, just look up and I'll be there!
Actually, I had a character named One-Up in a comic book I was writing after high school (like most things I did back then, it was never finished). The character was a cyberspace hacker, who "got his name from an ancient video game reference." I was RMFX (RM are my initials, FX is what I do for a living) on the old message board, so when the new board started up, and I had to create an actual login, I took it as a chance to update my alias.
1UP is an obvious game reference, is even easier to type than RMFX, and comes up pretty much first in the member list! Also, it fits perfectly with my competitive nature, you know, my drive to "one up" everyone else...
I've yet to see anyone use it, so...1UP.