Unlock-TMobileG1.com are the first people that has been able to unlock the T-Mobile G1 phone.  They are selling the codes to unlock the phone for $23.  You will be able to use a SIM card from any country on any mobile network and it will work.

Loic Le Meur, founder of Seesmic swears by it too:
Here is how I unlocked my G1 Android phone in 10 mins check here how to unlock it. worked fine for me http://www.unlock-tmobileg1.com