A friend made me aware of this (thanks, Peter):
The Inside Deets on iPhone 2.0.2 and Dropped Calls
Sounds like plausible explanation.
It is not the network that is fault but the interaction of the bad
power control algorithm in 2.0 and 2.0.1 software and the network that
is at fault. The sooner everybody is running 2.0.2 software the better
things will be. Having seen the graphs the 2.0.2 software has already
started to make difference.