According to Computerworld in Singapore, a UK-based company claims to be closing in on an unlocking application for the iPhone, allowing the device to be used on other GSM mobile phone networks such as T-Mobile in the US and many others around the world (alternatively, you can go the contract-free, Wi-Fi iPod route that Erica found). The UK company is Uniquephones and their founder, John McLaughlin, says they have "engineers working around the clock in several countries" to break the system Apple has in place for locking down the iPhone to AT&T.
As far as McLaughlin's team knows, there is at least a two step process to activating an iPhone. Engineers have already been able to circumvent the SIM activation process so another carrier's card can be used, but their attempts to meddle with the iPhone's firmware so it can work on another carrier have failed so far. Surprisingly, DVD John, the Norweigan hacker responsible for cracking the CSS encryption scheme on DVDs, claims to have ventured beyond this barrier, only to return with the bad news that the device can't be used as a phone when activating with anything but an AT&T account.
The Computerworld article cites another side of this coin, however: even if McLaughlin's team is successful, their site and anyone else's could easily be susceptible to legal action from Apple, forcing a take-down of the code and forever binding the iPhone to the carriers Apple choses to partner with. Only time will tell, but I honestly wouldn't blame Apple or AT&T for stopping iPhone unlocking apps in their tracks. While I'm just as unhappy about carrier lock-in as the next guy who would prefer to roam the wireless seas in any direction he choses, Apple and AT&T have still put a tremendous amount of collaborative effort into developing the iPhone (let's not forget the alterations AT&T had to make to their network and software for features like Visual Voicemail), and outside of all the new contract signups, we really aren't sure how else AT&T is getting compensated from this deal. The iPhone is still Apple's product, and they chose to bind it to AT&T's network (for now), and as much as I hate to say it, these companies get to decide how their products are used.
Nevertheless, the race to unlock the iPhone should be an interesting one. Should McLaughlin's team succeed and are able to package an unlocking app for sale, he plans to charge $50 for each slice of the iPhone unlocking pie.