Here is what I did.

• Jumped into a cab. Ask driver where is the nearest AT&T store.
• Walk into AT&T store, hold iPad out in front of me and ask person behind the counter "I want to get this on the internet with a pre-paid SIM".
• The lovely assistant (photo below) replied, "No problem Sir".
• She took my iPad, ejected my existing SIM, inserted a new AT&T one and waited for the activation screen to appear.
• Followed the online activation screen entering my credit card details for my Australian credit card (they recommended not using an AMEX as it can have issues with the region check). I selected a plan of 2G for 30 days for $25 USD.
• It started working.

Over the week I used 490Mb of data, way below the 2G I purchase. This was such an easy and fast process. There were about 3 other people in the store at the same time as me doing exactly the same thing. They even did nice things like tape my existing SIM card to the AT&T SIM caddy so I would not loose it and could easily store it.

Forget what you read on the Internet, its dead easy to get your iPad 3G service running in the US.

UPDATE 14/Sept : By default the plan does an Auto Renew so after the month it gives you another month. Before you leave bring up the service plan option and turn the auto renew off. If you forget to do this  simply ring 1-800-331-0500 and ask them to turn it off. It took me less than 2 minutes to get onto an operate and have this completed.

They mentioned that after 60 days the account will de-activate but that the SIM card is still good. So if when you return to the US you put the SIM in, you can bring up the service account information, re-enter your credit card details and be underway, no need to revisit an AT&T store. I will keep my SIM and try that on my next trip.

Rodos