Wow, this a major complaint not only on the iPhone but the AT&T network in general, this time from a corporate client:

My company had 120 handsets/aircards with a combination of Nextel and Verizon we consolidated all phones/aircards onto the AT&T network including the (4) iPhones. WHAT A MISTAKE to change from Verizon. Too many dropped calls, no signal.. I thought my problems were related to the fact that I was using an iPhone not true every one of the 120 mobile phones and aircards we have, have been declared useless by AMAZINGLY each and every handset user. Many of whom have personal Verizon Wireless phones that they end up having to use because AT&T cannot complete or maintain calls without dropping or being in-audible.

Keep sending in those iPhone related complaints and we will make your voice heard.