When I purchased my Sony PRS-900 Daily Edition reader, it came with a AT&T sim card installed. This makes it possible to browse the Sony Ebook Library store to purchase and download books. If you subscribed to any periodicals, they will download automatically every day, Sort of how the Kindle and Amazon. There are no fees or monthly costs to use this service and 3G is either active or standby mode (for better battery life) ready when you need it.
The iPad may have something similar and only allow connections to pay for data connection. Once you pay, AT&T servers will allow full access to the internet depending on what service you paid for. So weather or not you pay for access, the iPad sim will always be active as long as you have 3G service.
This is just my guess so take it with a grain of salt. We'll know for sure by Friday.
mokeiko
Out of curiosity, did you have to first create an account or activate the 3G service via phone or alternate Internet connection for the pay as you go service? I know that's how it is with the DataConnect Pass.