Right now the cell service is horrible. It is almost impossible to make calls. I am not being overly dramatic here. AT&T is really blowing this bad. I have a 3Gs and have never experienced such bad cellular service. They are clearly way over subscribed at this point.
I'm not having a problem where I live in Stockton, California. But I think with the addition of 1.5 million 3G iPads in the last two months... and the addition of 2 million iPhone4s, you may be on to something, especially in urban areas with lots of devices.
It's not signal strength to individual devices, its crowded cell towers handling too many signals, dropping connections as they switch around from computer to computer and tower to tower.