I imagine Apple would have a big say in what companies such as vodafone and optus can and can't offer... as per their agreement to sell the phone. 
 
I mean think about the App Store and the iTunes Music store, Apple will want it to be as easy as possible for the consumer to purchase as much software/music over the mobile network as possible. 
 
Also, limiting the data usage limits the usage of the iphone all together, making the phone less attractive for the consumer, making less people buy it.  
 
I'm just going to wait for the official word on plans direct from vodafone and optus.