One source says the problem lies squarely with Infineon's technology, which is fairly new and untested in high volumes outside a lab setting. Not only is the iPhone shipping in much higher volumes than other handsets, it's also gobbling up far more 3G minutes as owners use it to surf the Web, watch YouTube (GOOG) videos, and utilize other bandwidth-hogging services.
To date I've had no problems with 3G coverage on my iPhone but I haven't used it extensively due to lack of coverage in my area. I am, however, happy to report that a brand-spankin-new AT&T 3G cell site just went live about two miles from my house and it's helped greatly with coverage in and around my house.
Update: In related news Wired wants you to test your iPhone data speed at TestmyiPhone.com then register your findings on their global ZeeMap. (Tip: TUAW)