Normal touch-screen phones all have context sensitive operations, for example you are playing a game and no where on the screen can you accidentally press the "Fart noise" app button or the "Hangup" button -- because they just aren't there.
With Android there are those 4 or so required buttons (Home, Back, Menu, Call/Search) and moving them from mechanical buttons to capacitive you CAN find yourself in the middle of a YouTube movie or game where your thumb just barely touches the back button, closing the app and taking you home.
Or as another user mentioned, you are on the phone talking and part of your chin or face (if you try and neck-cradle the phone while doing something) may hit the hangup button, menu button or Home button to suddenly start activating other things on the phone, ending or interrupting your call.
It is absolutely MADDENING.
The iPhone never suffered from this because it always had the one single mechanical button and the proximity sensor was good enough that when it got close enough to your face, all input on the pad was deactivated. Most of the Android phones that have mechanical core buttons don't suffer from this either for the same reasons.
It's once you get those damn touch-sensitive buttons on there... oh boy, there are a few times a week where you just want to throw your phone right into the ground and wish it a happy birthday.
The difference, in my opinion, is that there is on-screen feedback when you press a screen button, vs a capacitive button which is just a backlit touch element. I agree with your solution to link their active state to the software unlock, but in practice it seems to not be how devices have been implemented.
I supposed this depends on the design/location of the capacitive buttons, but speaking for myself, I haven't really had the issues you mention in your post.