I do not drink coffee, so I don't know how the wi-fi works at Starbucks. However, to my knowledge, you must have AT&T service to get access to their wi-fi hot spots. The reason your iPhone picks up the wi-fi right away when you go into Starbucks is because you have a phone/data service plan already in place for your iPhone with AT&T. The use of AT&T wi-fi hot spots is included in your plan. In order to use the hot spots with your iPad, you'll need to have the 3G service active (either 250MB or unlimited). Wi-fi is typically faster than 3G, so that would be the benefit of using the hot spot. Battery drain is also reduced when using wi-fi versus 3G.
Oh, and here is the documentation/verbiage from their website
AT&T Wi-Fi
"AT&T Wi-Fi Basic service is included for the following AT&T customers:
High Speed Internet Customers:
Customers with AT&T High Speed Internet, AT&T U-verse High Speed Internet, or AT&T FastAccess? DSL?all speed plans included. No ordering required! Simply use your primary AT&T high-speed Internet membership ID and password at any AT&T Wi-Fi Basic Hot Spot. Learn more at
DSL Providers - AT&T High Speed Internet Access - Broadband Internet - Residential
AT&T Wireless Customers:
Smartphone customers with selected AT&T Wireless Devices and 3G data rate plans including the Smartphone $30 or higher rate plans (excludes Smartphone Enterprise 4MB plans). Qualifying Wi-Fi enabled Smartphone are listed at
AT&T Wi-Fi on Windows Mobile? Smartphones | AT&T wireless services
All Apple iPhone customers include AT&T Wi-Fi Basic service.
AT&T BlackBerry customers with a $30 or higher BlackBerry rate plans (excludes BlackBerry Enterprise 4MB plans (Requires a qualifying Wi-Fi enabled BlackBerry.) Additional restrictions apply. Learn more at:
Cell Phone Plans and cellular service - Wireless from AT&T
AT&T LaptopConnect Customers:
LaptopConnect customers with DataConnect service plans of $59.99 per month or higher. Learn more at:
Get Started - Wireless from AT&T
Additional restrictions apply. Subject to applicable AT&T Wi-Fi Basic terms and conditions. See
AT&T - AT&T Wi-Fi(SM) for further details.
Note: International Wi-Fi access through AT&T Wi-Fi roaming partners is not include with the above devices and rate plans. See
AT&T Wi-Fi for more information."