Actually, I don’t know if you’re contract has expired yet, I was just sharing the same information I got while standing in line for 7 hours at the Apple Store when the iPhone 3G came out on the 11th; you didn’t specify in your question that you currently owned an iPhone (although now that I ck’d out your profile I see your an expert on iPhone, but I digress)...
As for the details of the signup, I too had a 1st gen iPhone, and since that one was not subsidized, you and I paid full price for it up front (along with the $100 rebate, which was a one-time deal); therefore, you (like me) do not have to pay the premium for upgrading, nor do you have to wait for the contract to expire.
The whole point of a subsidized phone is that as an “incentive” to the customer and as a way for the carrier to ensure they have a customer for at least 2 more years, they pull this “discount up front” nonsense by giving you the illusion that you’re getting a discount, which you’re not; in the case of the iPhone 3G, you’re actually going to be paying an additional $15 per month for data fyi.
Describing this arrangement, I would liken it to the case of buying a new car from an auto dealer, except the car dealer would always give you the option to pay cash in full at the point of sale. In this case, it would be somewhat analagous to the car dealer requiring you to put a 50% down payment, and requiring you to finance the difference for the next two years (a two-year finance contract, with the data bump basically being the interest).
Apple and AT&T aren’t giving you that luxury of paying all in cash , insisting that when you upgrade, from that day forward, you will be AT&T’s customer for the next two years. That’s why their saying that even though you’re in a 2-year data plan, you can upgrade to the iPhone 3G (the current 2-year plan you’re on doesn’t include a subsidy for the iPhone you currently own).
Hope that helps.