It really screwed up my iTunes account. I have 2 iPhones that I was using to update, etc. with, and when Game Center came out, it only takes one email address per Game Center account. After changing one of the email addresses (not the primary) it switched, and the secondary became the primary.
I almost lost $25 I had from an iTunes gift card. ... No use for it now. Wish I hadn't set it up.