In the U.S. changing mobile phone carriers isn't trivial. All four major US carriers require a two-year contract which is backed up by a hefty early termination fees of around $350. But hey, at least you can upgrade your phone at the halfway mark, right?
Not so fast plucky...
Update: VZW confirms the 20 month policy in its wireless FAQ "You are eligible redeem your New Every Two benefit after fulfilling 20 months of your two year term."
This means that if you get a VZW iPhone this Tuesday -- and Apple continues its current pace of annual iPhone releases -- you won't be eligible to upgrade to the iPhone 5 in June 2011 (that's only 5 months out), or at the launch of the iPhone 6 in June 2012 (17 months).
You'll be eligible for a subsidized iPhone again three months later, and you'll be able to finally pick up the iPhone 6 in September 2012, but at this point details of the iPhone 7 will start to trickle out and you'll probably want to hold out for that.