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I think I shot myself in the foot. I bought and iPhone 4 within the first month of its release.
Over the summer (since that was when I anticipated a new iPhone would be announced), I checked my eligibility and I was eligible for a full upgrade.
Now, however, my upgrade eligibility shows up as 3/12/2012.
I assumed it was a mistake at first, but then I remembered that when my girlfriend moved to Ireland last month for grad school, I changed my plan from family to individual. That must have reset my eligibility. Damn!
Otherwise, I was planning to upgrade since selling your old phone almost always means you'll break even IF you're eligible for a full upgrade discount.
I would expect that buying now would mean that you're eligible for the very next iPhone, as has been the case for the last few releases.