I've owned an iPhone 5 for a few weeks now and, soon after starting to use it, noticed my cellular data usage increasing by a noticeable amount. It wasn't to the extent of the recent Verizon problems (i.e. it was, when extrapolated, a few hundred megabytes extra per month rather than the few extra gig that hit the headlines).
This weekend I did a number of tests to try to identify the problem.
In a nutshell, with the phone plugged in and charging and Wifi on and reliably-connected, there was a constant background cellular data trickle of around 60-70k every 10 minutes or so. Multiplied out over a month, this is around 300Mb, which was exactly consistent with the extra usage I noticed soon after starting to use the phone). This was with all location services disabled, all notifications disabled, all iCloud features disabled, iTunes Match disabled and all Mail accounts temporarily disabled. Turning all of these back on made no difference at all to the background rate of cellular data usage. I know the Wifi was reliably-connected (i.e. not dropping out and using 3G instead), because, during one test, I was streaming a movie with AirVideo, which only works over Wifi. During this test, around 900k of cellular data was used over a couple of hours.
In my view, there should be NO cellular usage at all when Wifi is enabled and connected, especially not 300Mb per month.
I called Apple support who, after suggesting that I disabled iTunes Match, iCloud etc. (which I'd already done) were mystified and could offer no further advice or help.
I found the following:
... which was seeming to suggest that some push services will always use 3G in preference to Wifi (!!!)
However, during my testing I'd already disabled all notifications. Or so I thought...
3 of my Mail accounts are GMail and are set up using the Exchange Activesync method (which Google is, apparently, about to stop). All of these were set up to push email (because I love push email and it was working really well). During my testing, I'd disabled these accounts by simple toggling off the Mail sync on the accounts.
However, as soon as I set all the fetch schedules to be "Manual" the cellular data trickle stopped.
In short, regardless of whether or not my phone was connected to Wifi, the Mail push connection was always pinging data back and forth via 3G instead of Wifi. I would call this a bug. My iPhone 4 (running iOS 6.0.1) certainly didn't do this. I always obsessively monitor cellular data usage and I would have noticed this. I also tested a colleague's iPhone 4 (6.0.1) this morning and it did not exhibit this problem.
I'm now using the native GMail iOS app, which happily refuses to use 3G when connected to Wifi.
In retrospect, the statement:
"When connecting to APNs, iOS devices will use the cellular data connection if it's available. Only if the cellular connection is not available or viable will the device switch to Wi-Fi for APNs connections."
...from the Apple knowledge-base article TS4264 (link above) appears to be either out-of-date or pure hogwash. Once I'd disabled push email, my cellular data usage was exactly zero over a couple of hours (even while receiving notifications from a number of apps, including GMail).
I agree with everything you said. However, I don't want any cellular data usage while I'm connected to Wifi (which is more often the case than not for me). This is definitely a change in behaviour between my iPhone 4 and iPhone 5 (which might be related to changes between iOS 6.0.1 and 6.0.2, but I can't be sure about that).
Edit to say: I don't want any cellular usage while I'm connected to Wifi and, in particular, while my phone is charging.