If you can get the 3G iPhone for $199, that will be a good deal compared to the expect prices which are predicted to be $200 or more higher.
In order to get a $200 subsidy from AT&T, you will have to sign a multi-year contract with AT&T...will their service fees be competitive? That is the real question. Typically, service companies will subsidize because they will get the rest of the cost from you over time.
This is the reason that many people have chosen to purchase unlocked iPhones so that they can decide who the best carrier will be for their money.
If you want to read more about how to get the best prices on an iPhone, try:
http://techland.blogs.fortune.cnn.com/2008/04/29/att-to-cut-the-price-of-apples-new-iphone/
or
http://searchwarp.com/swa299984.htm
I hope this helps.