As noted by a number of users on AT&T’s discussion form, AT&T’s iPad 3G plan will not accept a PO Box address for activation. As described by one poster:
“When I enter all the information on my iPad for the AT&T wireless plan such as my PO Box number it tells me to enter my physical address. However, my credit card bill is sent to a PO Box so the address will not verify. I have tried entering both the physical and the PO Box. However, anytime any field has a PO Box or anything similar (i tried USPS Box) it will reject it.”
In order to work around this issue, you can use your home address for the billing information street address, but enter the PO box’s zip code for the billing zip code.
Alternatively, contact your credit card company and ask them to change the billing address to your current home address.