You know, I'm sure the problem is there's like 20k people all hammering the server (which can actually take it, I suspect), and that server is contacting AT&T's server to see if people are eligible to upgrade. I'm sure it's this server that is dying, and cannot respond to the requests in time, therefore they time out. Everyone refreshes and does it again. It'll only start working once people a) start getting their orders through and stop hammering the server or b) give up and try later, thereby stopping the hammering on the server.
You know AT&T isn't going to fix it, and there isn't much Apple's team can do...
I'm sure Jon Stewart would call this Clusterf*ck to the iPhone 4.