Here's my situation. I used my girlfriend's completely separate AT&T account to get my iPhone 4S. Her line was eligible, so she upgraded, I paid, and the AT&T guy immediately swapped the lines for us to get the phones both working on the correct numbers. All is good, except that my data speeds suck. I get about 1.3Mbps down, and 1.5Mbps up. I've tested several times over the last 2 days and never get any better than that.
But I log into my att.com account and it shows that I've got a 4S data plan. So I'm not sure what's going on exactly, but I feel like I'm not getting the proper speeds.