Consider this. Let's say Apple does something along the lines we're predicting, and sells their phones. Before we plunk down our money, we go around to the various cell carriers and inquire if they'll let us bring our phone to their network. They say either "NO!" or "Not at this time."
The only mobile carriers in a position to do this are the cdmaOne/CDMA2000 ones (Verizon, Sprint PCS, etc.) If Apple makes a GSM or UMTS phone, the carrier has little or no say in whether you use it. T-Mobile and Cingular will, by next year, be running both types of network in the US, and both already run GSM.
The real influence the cellphone companies (at least, the ones not stuck in the 1980s as far as their network infrastructure goes) have on phone purchasing is the ability to subsidize phones that fit their model. This, in practice, usually means rebranding. Cingular is pretty good on that score and rarely insists on more than some ugly logos printed on the phone (unfortunately their network is not the greatest GSM implementation in the world.) T-Mobile, in my experience, is somewhat worse, though not always for bad reasons. For example, they'd probably insist on "My Faves", a proprietary five person phonebook, being grafted on to whatever UI an "iPhone" has, in return for any substantial subsidy.
The fact Apple can't expect carriers to subsidize their phones is one issue they have to deal with. I'm more concerned though with Apple becoming a minority player, with its phone tied to a music store whose success was, in major part, to do with the giant marketshare it had, and thus Jobs's ability to force the labels to compromise on prices.
What would make absolutely more sense is for Apple to simply start up their own network. They've already acquired some assets in this area, haven't they? So why not bide their time until they can really roll the thing out? And since it is relatively common practice for cell towers to have more than one (sometimes several) carriers' equipment mounted on them, Apple could buy into who's-ever network they needed to get one of the "lesser third party" broadcast equipment sets that's already out there among the masses.
Apple would need not merely infrastructure but spectrum to actually start a carrier. They have neither.
Purchasing a carrier is an interesting pipe dream and would terrify the crap out of most shareholders. Mobile telephony is a long term thing, with very little return on investment yet for most people who've invested in it. It's not even a good time to get involved, most companies are rolling out 3G networks and 4G, in the shape of WiMAX, is already being released in some areas.
Were they to do the carrier thing, the best they could hope for would be to be an MVNO. This would be a major change of business model. It has so many ramifications I don't know where to begin.