Not AT&T be a virtual carrier but rather handle certain aspects of operating the iPhone, iPod Touch, and the rumored Tablet. Currently there is probably some aspects handled by AT&T. As it is any data, authentication, customer info, iTunes purchasing, etc... is all going over AT&Ts network but then having to authenticate with Apple. I imagine Apple will use this data center to retain a certain amount of control over the iPhone users but allow customers to use all mobile networks have access to the iPhone... from their they may do something like you're suggesting, open up a VOIP service.... considering all the fall out with the Google CEO leaving Apple's board, the blocking of the Google Voice app (which are allowed on AT&T and the Blackberry) seems to be Apple is scheming something....

Why? AT&T provides bad service, customers are unhappy with them. Apple has the real goods- the iPhone. None of us non Apple or AT&T employees know exactly which company handles which aspects of iPhone usage other than the basics. Apple, the hardware, AT&T, the wireless towers. Yet there are details to be taken into consideration- the authorization of phones, the security, the iTunes purchasing, this is all data going through AT&T currently. If Apple owns their own data center they can most likely work with any mobile carrier they desire.