Adar, I got it. But what is the reason/purpose to turn Mobile Data off?
I turn the mobile data off purely for battery saving. When I am at home, I get only 1 or 2 bars (despite having a tower literally 2 blocks from my house). This causes me to use an inordinate amount of battery. OK, that may be a bit of hyperbole. However, it is a noticeable increase in battery use if I leave mobile data on. Technically, it shouldn't really matter. When I have the Wifi on, it shouldn't be USING the 3g, so my battery savings are minimal. Due to the behavior of the 3G coming on when Wifi goes to sleep, it DOES tend to end up eating my battery more than I want it to. Now, part of it is probably due to background data, and the various polling that it does for things like email and Facebook.
It isn't really a huge problem per se. Just that the behavior is not what it used to be, and I can see no logical reason for it to behave this way. mobile data should NEVER turn on by itself when I have specifically turned it off. In my case, it is simply a battery saving ploy as to why I turn the mobile data off. For others, it may be due to data usage, or preventing annoying notifications while sleeping, as someone else suggested. Regardless of the reason that someone turns off mobile data, there is no excuse for it to turn itself on without user interaction. I haven't tested this yet, but think of this as a possible situation. What happens if I put the phone in airplane mode, and after it goes to sleep for a while, it turns the mobile data back on? Wouldn't you see that as being a problem?
All in all, this is a pretty minor annoyance to me. Mostly, I was just curios if anyone else has seen the same thing, or if it is something messed up with my phone. I am not rooted, and have no plans to root, so I have no special apps that might be doing it. I don't even have anything like Juice Defender running that might explain wonkiness with radios turning on or off. Possibly something got messed up on my phone when I did the upgrade. It is possible that a factory reset will make the problem go away. A battery pull has had no effect so far, and I haven't found any other setting or app that could conceivably cause this behavior.