If you received an error message during the sign-up process for AT&T service on your iPad with Wi-Fi + 3G, you might want to check your credit card bill. At least one iLounge editor has seen his card double-billed for one month of unlimited service after the initial sign-up process hit a snag. It is unclear how many iPad with Wi-Fi + 3G users may have experienced a similar situation; if you’ve had trouble with your AT&T iPad billing,…
