*Update* Apple now offers an iPhone Upgrade Eligibility Tool where you can check for your upgrade eligibility for all 3 carriers: Verizon, Sprint, and AT&T. It’s a fast and easy tool.
If you want to check if you’re eligible for AT&T’s subsidized pricing for the new iPhone, the easiest way if you already own an iPhone is to dial *639# and tap Call. Another way for AT&T Wireless users to check is to log in to ATT.com and click the Check Upgrade Options link on the rightside of your account view:
A third option is to go into an AT&T Wireless store and ask them to check your upgrade status.
For the iPhone 4, AT&T Wireless has announced that it has moved up upgrade eligibility by up to 6 months for qualifying customers.
Check for iPhone upgrade eligibility at Apple.com