Well, the details on the $100 iPhone rebate are here - looks like a pretty easy process for most people:

http://www.apple.com/iphone/storecredit/

However, if you’re like me, and live outside of the US, the elation of finally getting your phone working on a non-AT&T GSM network might be slightly dampened. According to the fine print on the above page:

(7) iPhones registered to customers whose addresses are outside the United States

*sigh* At least I got mine refurbished!