Do I need WiFi access to sign up for a 3G data plan?

For the first time today I found myself with my iPad with no WiFi so I tried to sign up for a 3g data plan. I saw 4 bars for AT&T but when I selected View Account it said it could not contact the server. I tried for hours in different locations without any luck. As soon as I connected to a WiFi network and selected the View Account I was able to sign up for a 3g data plan. This leads me to believe that I need to be connected to WiFi to sign up for a 3g data plan. I can't believe that this is true because when you need the 3g data plan is when you are not near WiFi. Is this the case or am I doing something wrong?