The T-Mobile G1 can now be unlocked. A site dubbed Unlock-TMobileG1 offers to unlock your Android phone for a fee - a not very "open-source-thing" to do.
The guys over at Android Community are still happy about the unlocking possiblity, even if it doesn't give you all the features.
The unlocking requires $22.95 and sending your phones IMEI code to the unlockers. You will get an unlocking code in return that you need when you put in a new SIM card in your G1.

You can use the unlocked G1, but apparently you cannot log into Gmail and other Google services, rendering the G1 half as useful since all data is in the G Cloud.
More details on Android Community and on Unlock-TMobileG1.