I definately have the problem despite living and working in the Minneapolis metro area. The variability of signal strength ranging from "Searching" to 5 bars is much more apparent on my 4 compared to my 3GS. While I avoid touching the "spot" I still tend to get dropped or missed calls.
I am able to visit some areas of the city where I get 5 bars and I am unable to lower them by touching the spot. I assume I am near a cell site. In other areas I can fluctuate from 1 bar to 5 bars depending on the direction I am facing. I assume I am between two cell sites and connecting to one or the other. This is also apparent when I drop a call and when I attempt to reconnect I have 5 bars again. This type of volitility in getting a connection may be due to the fact that AT&T has too few cell sites in parts of my city and is why others may not be experiencing the issue.
I read in another thread where someone turned off 3G on their phone and they were getting better connections. I tried this and the incidence of dropped and missed calls was dramatic. With 3G on while at my desk at work, I get 4 bars yet I miss 50%+ of incoming calls. When I turned 3G off, I get 5 bars and I have not missed a call in 3 days. While this is a short period to test, it is remarkably different with 3G off. As a test today, I turned 3G back on and both test calls were not picked up by the iPhone.
I believe that the issues we are observing are a combination of the radio that Apple has in the iPhone and possibly the software that manages it as well as the AT&T network both from a cell site placement perspective and possibly the load on their network.
I think my phone in the AT&T network area that I am in has a really tough time passing from once site to the next while standing still or mobile. This may be why making and receiving calls is so problematic. Where exactly the problem lies is something that I hope Apple and AT&T can resolve, soon.
In the mean time, I will keep my iPhone 4 because I like the size & feel and some of the features only available on the 4. I can live with 3G off because I use wifi at home and work.