I'm pretty sure that if you have an iPhone on ATT you get free WiFi at Starbucks, PERIOD. Not just on your iPhone. In other words, you could go into Starbucks with your notebook and still use free WiFi.
I'm unfamiliar with the signon procedure, because the only times I've done this have been on my iPhone and using the Easy WiFi for AT&T app.
Has anybody tried Easy WiFi on the iPad? You need to plug-in your AT&T iPhone number. Oh, maybe not... I seem to recall that the way this works without the app is that you text some number, then they send you a code via text message and you have to plug in the code on the login screen at Starbucks. Easy WiFi just automates sending and receiving the text messages, and then logging-in to the WiFi node.
OK, so do it the hard way: you'll probably have your iPhone in your pocket anyway. So, you send the text, get the reply, and you enter the code ON YOUR IPAD rather than on the iPhone.
Edit: you don't enter your phone number in Easy WiFi. It's already entered for you, guess it gets it from the phone. So, that way is out.
The AT&T web page for WiFi Access says this:
"Users with a Wi-Fi compatible device, a qualifying AT&T data plan and an active account in good standing can relax and access music, email, and Web browsing services with their favorite blend in hand from the comfort of their nearest location."
If you have an iPhone on ATT with the iPhone data plan, you have a qualifiying AT&T data plan. An iPad is a Wi-Fi compatible device, so I think you are good to go. Nothing there that says you have to use your iPhone to access the WiFi.