Going MVNO would put them in the awkward position of competing with their own suppliers. Then again they're in this situation with Samsung and it hasn't been fatal yet. The one thing that makes this interesting to me is that they are now shipping handsets that will work on multiple carriers in almost every market. This would let Apple brand the service as their own but pit the carriers against each other behind the scenes to get the best deal on connectivity. A customer's phone might switch networks from hour to hour and they'd still pay the same bill and see an Apple logo in the upper left.