Not sure if this is just a Premier thing or not, but this is my experience.
What I noticed was before I got my iPhone, if I wanted to upgrade a phone, even at full price, the iPhone was not listed. Once I got an iPhone (Thanks to my mother upgrading her 6 month old 16GB 1st Gen to a 16GB 3G) and activated it to my account, if I choose now to upgrade any of the other lines on my account (3 total, me, wife, and my nanny), I can now see the IPhone as an upgrade option. I just upgraded my wife to the Refurbished 8GB 3G phone, Now, I had to pay the Full $199 (Seems us Premier people do not get some discounts) but I was able to upgrade her phone even though she was not eligible for a phone at a discounted price.
I think some here said it before, it seems if there is no iPhone on the account then a person can not upgrade to an iPhone till they are eligible. Once you have one on your account, then it seems you can upgrade to other iPhones at any time. You just have to sign a new contract each time.