Was a huge PITA ass for me. Had to activate two accounts (one with 8G, one with 16G), call AT&T and talk on the phone for 2 hrs to get it solved. The problem begins when the system recognizes you as having used the subsidized price to purchase the 8G, and won't let you purchase the 16G @ the subsidized price. Read all about my experience here:
https://forums.macrumors.com/threads/532747/
But I gotta tell you, I freaking love my 16G way more than my 8G although I had my 8G for 3 days lol.