Like Revelation78 mentioned, it depends.
If you pay full-price ($599/$699) for the iPhone in July (assuming they stay the same price as now), you won't need to renew your contract because AT&T didn't help you with the price.
If you do an early-upgrade in July ($399/$499), then yes, you will need to sign a new 2-year contract, because AT&T is giving you a $200 discount off the price of the phone, and you haven't even finished paying them back the $400 discount they gave you on the iPhone 3GS (assuming you bought it at $199/$299).