OK, secret cause of the ongoing iPhone problems, latest version: it's not, as reported earlier, the antenna. In fact, some crafty Swedes confirmed this with tests on actual phones that have been suffering problems, so hardware seems unlikely.
Now an anonymous source is saying that the problem arises from the interaction between the iPhone software and the actual cell towers themselves, with the phones improperly causing the towers to drain power and drop calls. This fits in with some of the observations that problems arose in high-call areas, as that's the scenario that causes the power drops. It also offers an explanation as to why the 2.0.2 update, which was supposed to fix the issue, didn't work immediately for most -- the problem is a result of many phones interacting with the towers, so even if you're your phone is at 2.0.2, there are lots of 2.0 and 2.0.1 phones out there wreaking havoc. This in turn explains why AT&T and Apple are so keen for everyone to upgrade now, even sending out text messages to that effect.
The lesson: network effects are awesome in any number of ways, but they can cause problems that spread along the network as well.