Okay, from the top:
1) When you buy another phone from AT&T that is either partially or fully subsidized, your contract start date is RESET to be from the date that you bought your new phone. It is not TACKED ON to the end of your current contract end-date. So if you had a year left on your contract, and you re-upped it, you would then have two years left, not three. A quick phone call to AT&T would confirm this.
2) If you already bought a subsidized phone -- ANY phone model...Blackberry, iPhone, cheap-ass Samsung thing: it really doesn't matter -- three months ago, then there is no way that AT&T is going to allow you to buy another subsidized phone already. You are going to have to pay full-price. Now, depending on their mood, you may luck out and discover that you are eligible for what is called an iPhone "Early Upgrade" price, which is $200 more than the $199/299 fully-subsidized price instead of the $400 more you would need to pay for an unsubsidized phone.
So you will either be paying $399/499 and resetting your contract date to 2 years from now, which means that instead of a 2 year contract you will have had a 2 year + 3 month contract (really not a huge difference between them), OR you will be paying $599/699 and not having your contract date reset at all.
Now, I'm willing to bet that after 3 months, there's probably no way in heck they are going to extend the "Early Upgrade" price to you, but I could be wrong, seeing how you have obviously been with AT&T for a while given that you had an original iPhone before you went with the Blackberry, and they may be willing to take that into consideration.
If they give you the Early Upgrade price, then I say take it. 3 months of extra contract for $200 off is a no-brainer.
Oh, and yes: you will have to switch to an iPhone data plan. And no, there is no way around that. The unique IMEI number that the phone broadcasts to the cellular tower tells AT&T what the make and model is. It is a federal felony to tamper with a cellular phone's unique electronic identifier (IMEI or ESN depending on the cellular system) for the purpose of defrauding a phone company.
-- Nathan