i bought two iphone4's on launch day and like the rest of you that pay less than $100/line, we're not eligible for upgrade until Feb/2012. However, we are eligible for the early upgrade for $199 + $250 plus the $18 fee. So for us, the iphone 5 will cost $467+tax each under the current contract in my wife's name.
I rather not pay that much so I came up with another plan. The early termination fee, by the time the iphone 5 comes out, should be around $175 give or take. ($325 - $10 for each month on contract) The new iphone 5 should then cost us about $374+tax each under my name as a new account. Selling the unlocked iphone 4 on craigslist should be enough to cover most if not all of that.
To get around the new account/new phone number issue, I will port our current AT&T numbers to google voice and never have to worry about giving out new numbers to friends/family again.
When you attempt to do this, they require your SSN. They will then run that for the credit check, they will see that you just cancelled your contract and probably try to put you back on the same contract.
The timeframe to be considered a new customer is 90 days I think.