Dan Gillmor blogs about a suit filed by a consumer group against mobile phone carriers which "lock" phones. The argument by the mobile carriers is that they subsidize part of the cost of the phone and therefore have the right to now allow customers to use the phone with other carriers. Dan makes some good arguments about why this may be a red herring. It will be interesting to see how this suit turns out.
In the mean time, a quick Google search will provide links to lots of people offering services and information about unlocking phones.
From a Japanese perspective, I'm quite envious that at least you're using open standards and have the option of unlocking phones. We can't even imagine using our current Docomo phones on any other network.