I have a few months left on my AT&T contract with my iPhone 4. I'd like to upgrade to the iPhone 5 but don't want to be locked in for another 2 years. Unlocked phones from Apple are $849 for the 64GB. $399 with contract. AT&T's ETF is $350 so I'd end up saving $100 paying the ETF. Is there some reason that I wouldn't want to take this approach vs buying the unlocked version from Apple?