Surely it's just a result of the bars being smoothed/some sort of running average of strength.

But if I move my iPhone around in the room, it seems to change pretty quickly.

To save power, the phone shouldn't be constantly trying to acquire a signal when it can't find one. It should try for 10-20 seconds, give up, then go to sleep, only waking every 30 seconds or so to try again, briefly.

Don't you guys remember Antennagate, where the following iOS update smoothed how fast the bars change? I bet if you did the same thing with a phone running iOS 2.0 it'd move instantly.

Most likely these are natural bar changes that are happening anyway. The glass or the "magic sticker that goes on your battery" just fools you into believing confirmation bias.

Another datapoint:

There's no cell coverage in my basement. Sometimes you'd get 1 bar, but it will drop soon afterwards.

After putting the phone inside a glass, I have 3 bars. I took it out, held it by the charm (is that how it is called? The little string with a little figure that you can put on most phones today) and coverage dropped to 0.

Works for me.

nope, its not, complete reproducable with 3 different phones.

The radiation pattern of the antenna is probably different enough with the phone in the glass to cause the network to switch it to a different cell tower altogether.

Also, "radio is fast" but signal-strength indicators are not updated at millisecond frame rates. :-P