this is an AT&T AND iPhone issue
My husband has been an AT&T user for over a decade. He never experienced dropped calls until we started dating and he was talking to me (I'm on an iPhone, he is not). We often get disconnected 2-4 times per hour as we talk during our commutes home. We have different shifts, but take the same routes home and we get dropped no matter whether I'm stationary and he's moving, vice versa, or if we're both moving. This also happens when we're on business trips - both stationary - him at home, me in a hotel - and we will get disconnected. The recurring motif has been the iPhone. When I talk with others who have AT&T but no iPhone, they only get disconnected when they are talking w/ someone who has an iPhone. The worst issue is when I am communicating w/ someone iPhone to iPhone.
IF this wasn't the iPhone and otherwise so awesome, I would have switched a long time ago... and frankly, I'm still contemplating going to another phone when my contract is up - because the dropped calls are so aggravating.
Coworkers of mine that have switched from Blackberry on AT&T to iPhone have reported an inordinant number of disconnected calls since switching to the iPhone, even though it's the same carrier, same phone number and same physical location of use.
My "assumption" is that the iPhone software is making some errant call to the tower intermittently (whether too high/low power request or other issue) at which point, the tower drops the call.
While my experience with disconnects are sometimes random, there are some places that either I or my husband will be travelling by, when we will experience a disconnect - a place where he never gets disconnected while speaking to others w/o iPhones... places I never got disconnected before having an iPhone, either.
This may not be just an AT&T issue. It could be when you are a certain distance from a tower (lower power or significantly higher power?) and/or the phone is experiencing a push of data, that the interrupt happens.
This has largely been the elephant in the living room that AT&T and Apple has been ignoring. I have not only not seen an improvement, I've seen the situation get worse over time - whether this has to do w/ an increase of iPhone use faster than the towers can keep up, OR problems w/ iPhone OS updates or a combination of both - who knows. They need to fix this already.