George Hotz has released what is, as far as we know, the first software-based mechanism for unlocking the iPhones running software/firmware 1.1.3 for use on unauthorized wireless carriers. Hotz is the Glen Rock teen that delivered the first hardware-based iPhone unlock in August of last year.
The new unlock method is not for the faint of heart and requires modifying your iPhone's baseband -- a procedure that can genuinely brick the device, rendering it (at least temporarily) non-functional. We certainly don't recommend it. There are step-by-step instructions here, which are Windows-based but can be easily modified for performance on a Mac OS X system. Although the steps are listed as for 1.1.2, they also work with software/firmware 1.1.3.
If you have trouble downloading the listed, necessary files, they are also available here.
If you need the iPhone 1.0.2 firmware (required by these instructions), here it is.