I activated mine in the store yesterday. The rep entered my Nexus 4 IMEI, which wasn't recognized. He chose an option called "customer provided device" and then he switched my data plan from the unlimited iPhone 3G plan to "Unlimited for Smartphone Personal HSPA+". I watched to make sure he picked the plan that was HSPA+ and Unlimited.
My AT&T account now shows this for data:
"Data Unlimited SmartPhone Personal"
"Data Plan for SmartPhones. Unlimited domestic data usage. Only available for upgrades. Requires customers to be on a "Gold List" Data Unlimited SmartPhone Plan prior to upgrade to 4G device. "
I deleted the wap.cingular APN and created the "phone" APN listed here and on XDA and rebooted the phone.
I have run the Speedtest app several times to verify my speeds, and depending on the area I'm in I tend to get 3Mbps to 8 Mbps down. I have seen it get close to 10 as well. That being said, I haven't had a chance to test it very much in different areas, but everything seems to be working just fine.
Hope this helps.