AppleInsider seems to have figured out the problem for the loads of iPhone buyers who were told during the activation process that they couln't port their current phone number to the miraclebrick. Seems the problem happens if the area code of your phone number doesn't correspond with the ZIP code of your billing address, a likely occurrence if you've moved around a bit.
FCC rules apparently require geographic harmony, even though carriers are usually willing to overlook the requirement when activating your phone. The software baked into the iTunes registration process allows no such wiggle room, however, so you'll have to fudge a little and use your old physical address at key points when registering.
How to port 'ineligible' mobile numbers to AT&T and iPhone [AppleInsider]
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