According to some reports, customers are unable to activate their phones at the store, but have to go home and keep trying over and over again to make it happen. Ditto for installing the iPhone 2.0 upgrade for existing handsets.
Part of the problem may be the unprecedented scope of the launch. Unlike the country-by-country launch of the original iPhone, the new iPhone 3G is rolling out in dozens of countries around the world, all at the same time.
Another part of the problem may be in the nature of the software upgrade. It's a big upgrade, Apple did not engage in a widespread beta test of the iPhone 2.0 software or of the iTunes 7.7 software for Windows and Mac. That's a lot of moving parts, which may not have been tested very thoroughly.
Sadly, these sorts of glitches are becoming less and less surprising to Apple customers. The Mac OS X 10.5 "Leopard" upgrade, which came out last October, was also marred by bugs, many of which weren't remedied until the 10.5.2 release. Even now, some say that the latest version of Leopard, 10.5.4, isn't as solid as Mac OS X 10.4 "Tiger."
Apple has proven that it can pack customers into stores, as today's queues attest to. It hasn't yet proven that it can handle all those customers, however. In other words, it may be that Apple's marketing is better than its technology.