Steve Jobs has said that he'll be fighting against those companies that allow you to unlock your iPhone. Darn.
As with most security measures, it will be a back and forth relationship between those that will manage to unlock the iPhone, and Apple. With the emergence of companies that are offering iPhone hacks that let you tap into another SIM card, stopping these hacks has become an issue that Apple has needed to address as it gears up to launch the iPhone in Europe.
So services like iPhoneSIMFree may no longer work on your handheld, if Jobs has his way. Some have pointed out that hacks won't be as popular as they're made out to be, as you'll be charged higher rates for utilizing certain functions on the iPhone when being used through another wireless carrier, and you'll no longer have access to many of the services that have been built into the existing plan with the iPhone.
This is especially true in Europe, were O2, the iPhone retailer for the UK, has struck a deal for several service offerings. In the US, however, there's a new device that's expected to be released, called the Touch. This will be an iPod with Internet access, giving you some of the features of the iPhone--without the phone. The hope is that those wanting these capabilities without signing long term contracts or switching wireless providers will find the Touch to be of value.