Well, this username I have on this site is a combination of two identities I've acquired. One (Aby) from Guild Wars, and the other (Z) from Xbox Live. My other, new one is either aZn or Death (maybe both).
I see usernames as just another way to identify someone. In the end, a username (or perhaps the shortening and... nicknaming of the nickname) can end up truly becoming part of you. It's funny, as 'Z' is my middle initial, so it works out quite well.