When the iPhone was first cracked, hackers wanted insane sums of money to unlock the phones for users interested in breaking free of AT&T's tethers. Eventually, the iPhoneDevTeam cracked it and offered the unlock for free. I expect that the people behind Unlock-TMobileG1.com won't be the only ones to figure out how to get at the G1's locking mechanisms and pick them.
Until someone does, G1 users interested in unlocking their device will have to fork over the $23 and hope that it works. The Android Community reports, "After placing an order including your IMEI code you will receive an eight-digit unlock code for your device. After you receive your unlock code you insert a non-T-Mobile SIM card and the G1 will prompt for an unlock code, enter the unlock code provided to you and hit unlock. You will then know the device is unlocked with a notification that reads 'Network Unlock Successful', it is as simple as that."
There is one big caveat. The G1 will be unlocked, but it will also be mostly unusable. The Android Community folks tested it out. As soon as they put in the new SIM, they were logged out of their Google accounts and were unable to log back in, even with the proper credentials. This means Gmail and the Android Market won't be available once the G1 is unlocked.
If you ask me, that sort of negates the entire appeal of the G1.