I can answer this pretty clearly for you (I hope) since I work at ATT...
The iPhone doesn't have a discount with the two year plan because of the contract with Apple. My assumption is that Apple feels you are getting such a good deal with the unlimited data ($20 a month for internet, email, visual voicemail, etc.) that a phone discount is not neccessary (might also be why they don't allow company discounts on the plans either).
At any rate, it's an awesome phone, so if you do take the plunge, enjoy

Oh, and PS: You don't need to be eligible for an upgrade to switch to the iPhone... it automatically renews your two year contract when you activate the phone through iTunes.

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