Cell phone antenna boosters
I think I might be able to add some anecdotal evidence to what you just said. I have one of those new T-Mobile Sidekick GPRS phones and my coverage is HORRIBLE where I live and widely variable elsewhere. I had one of those stickers on my old Nokia phone (don't remember the model it was so old...4 years!) and I couldn't tell if it was really doing anything or not. I thought it helped a little, but not enough to really know for sure. My coverage at home was very poor even with my Nokia.
On my T-Mobile Sidekick, coverage was virtually non-existent. I got faint signals now and then, but ever since I transferred the same sticker I had on my old Nokia over to the back of my Sidekick, the thing has been able to receive a weak signal most of the time at home. It's definitely doing something, unless there's just a sheer coincidence that T-Mobile upgraded a lot of equipment the very hour I put the sticker on.
If one of them seems to work for me, then would a second one add anything? Also, does it really matter which direction is up? I really have no clue which direction is up on a device like the Sidekick.