Meanwhile, CNET is reporting that T-Mobile in the Netherlands has a blog post (original Dutch) suggesting that the problem is "...a hardware / software specific issue of the iPhone itself," while a Vodafone spokeswoman in Australia told the Sydney Morning Herald that the problems were specific to the iPhone and not their network. Apparently, the problem was compounded by Apple's famous secrecy, with the iPhone 3G only released to some carriers for testing a day before it was released.
It's not that surprising that a new product would have growing pains. At the same time, it's hardly the case that the networks are as fast and extensive as we would hope. Hopefully, future firmware updates will improve the situation (if it's primarily a software issue). Otherwise, Apple may have to change the radio chip supplier for future iPhones to bring performance in line with other 3G smartphones.