iPhone reception
Actually, you'll come across this same behavior with all types of cellphones, in the right situations.
Your body acts like an extension of the antenna of the phone when you're holding it. (That's one reason you'll actually see it printed in the instruction manuals for some phones that you need to make sure to hold it in your hand a specific way when using it.)
If you've ever played around with antennas before on home stereos and such, you might have noticed that simply making the antenna wire longer doesn't always improve reception. Just as often, it makes it much worse than what it was. Same deal with cellphones. The radio waves are a certain diameter around for a given frequency they're broadcast at. An "ideal" antenna is supposed to be either the same length as the full diameter of one of those waves, or an even division of it.
Obviously, there's some leeway with all of this. (Otherwise, you'd have to have a different length of antenna for each radio station or TV channel you wanted to watch!) But the point here is, when your iPhone is receiving a relatively weak signal, it might be managing to receive a few bars of signal, sitting on your desk in the right place. But by picking it up to use it, you make its internal antenna sub-optimal, and then that weak signal just doesn't come in too well anymore (so down to 1 bar).
With better signal strength, your act of holding the phone doesn't put enough of a "dent" in the reception to really register as "fewer bars"....
Im not sure if my issues are due to iPhone design or AT&T, but when my phone is laying on a table i get 3 or 4 bars and as soon as i touch it the bars drop to 2 or 1, if i had to guess i would say the antenna design is poor and is the problem