Most MVNOs are second tier resellers, sometimes owned by the major carriers, designed to appeal to buyers looking for cheap rates. APPLE could by airtime from all of them, in big enough chunks, to demand top tier service and manage connections to maintain service levels without having to roam. I doubt they would price like the current MVNOs but focus on service and unlimited data to make the ancillary products more compelling. For example, sell HBO through iTunes and offer it on mobile devices as well as AppleTV. No need for a DVR as you could stream and d/l content; and thus change the whole competitive landscape relative to cell phone and cable markets. If they pull it off you cut cut the cable and inernet access so you TCO could go down even if you pay more for cellular service. It could be a brilliant strategic move, or if it fails, only burn a little of Apple's cash.
What is the incentive for Verizon and AT&T to sell Apple large chunks of their capacity at discounted prices?