Make sure you call. Don't try to do it on the website. Like many others, I used the upgrade eligibility from a different line. As AT&T's website shows the new iPhone 4S on my line, I tried to update to the 4S Unlimited data plan. Unfortunately, that didn't work. Instead of processing the update, it disabled my data plan completely.
When I called in, they told me their system reflected that I had been on an iPhone pay-per-use data plan since late 2007. It took a couple transfers of the call and a supervisor to get the unlimited data plan restored. They kept denying the fact that I had ever had it, but the supervisor finally reviewed last month's bill to discover that it was actually there.
Before the phone call, I was getting download speeds of 0.5Mbps and upload speeds of 0.2Mbps.
After the call, in the same position, and having the same number of signal strength bars, I am getting 2.60Mbps download and 1.18Mbps upload speeds.
It may just be the change in the weather or usage conditions in the area, but it seems to be better for the moment.