Thanks.  That's helpful. 

That being the case though, if someone finds they're  using more data than they think they should be, or more than they're willing to pay for, the system of reporting makes is impossible to tell what's using that data.

How about the issue of whether or not an iPhone 4, with IOS 5, uses cellular data when it's asleep but charging and in range of wifi?  Is there a definitive answer to that puzzle?