These claims were quickly quashed by Apple today.
What strikes me as being so weird about this news is that Apple have yet to give any indication of having a leaning towards their own network, and piggybacking on someone else's network just seems so odd I'm amazed that anyone believed this rumour!
Apple is a premium product company. Their products are all firmly priced and everyone knows this.
Apple has a good reputation for producing great products that "just work", so venturing into the uneven and treacherous realms of MVNO operations just seems like a bad - and totally unrealistic - move for Apple.
In the UK, I can't name one network of the four remaining that's great. MVNOs that piggyback on these networks tend to be less "reliable" still.
I can't see Apple associating itself with this sort of situation, as it is totally out of character for the company and not only cheapens the product, but when it doesn't work it'll harm Apple's reputation.
Needless to say, even if this news had turned out to be true, I doubt that Apple's MVNO would be cheap.