Don't buy in California, Massachusetts, or Rhode Island.
If you are in one of those states and can cheaply get to another state, or happen to be passing through another state on business or to visit family, you can save $40 or more if you purchase your phone out of state.
Why? The answer is in small print at the Apple store and--possibly in different words--on some mobile provider sites:
In CA, MA, and RI, sales tax is collected on the unbundled price of iPhone.
In other words, you might be getting a great deal on the phone but these states charge you sales tax as though you did not get a great deal, and that's a bum deal.
Consider that the Apple iPhone 4S series has unbundled prices of $649, $749, and $849 for the 16MB, 32MB, and 64MB models respectively. That means a sales tax of 7.75% on the 16MB 4S you buy from AT&T or Apple for $199 comes in at $50 versus the $15.42 you were probably expecting. That's sticker shock if you have not been through this process before.