Yes, I am using the i9100 on the non smartphone data plan. I gave AT&T the imei of my old Moto V180 and it shows up when I log in onto my AT&T account. I hope it stays like this forever but in reality, I believe it's only a matter of time before AT&T catches on
15 dollars for unlimited data, or combine with 20 dollar unlimited texting + 10 unlimited data it's not bad either. I burn through 5-6 gigs of month and have been for the past 8 months since I bought the i9100.
Check it out, here are a list of phones that work on AT&T.
http://forum.xda-developers.com/showthread.php?t=1446373