Or you could always have a bash at the Rebel Simcard (rebelsimcard.com) which is dirt cheap. I have no connection with the company other than someone who has used one to unlock a couple of iPhones. The same simcard will unlock a G1 as well as no end of other handsets.
Rebel have two versions - one which involves punching a hole in the original SIM and another that does not - I think both will work in the G1.
What is nice is that they also do regular software updates to the simcard - so as they improve their technique you can update the simcard if you purchase the programmer with it. I paid £25 (UK) for two simcards, two [iPhone] screen protectors and a programmer, delivered.
There are demo videos showing how to do it. The G1 demo looks a breeze - slip in the Rebel Simcard with the original SIM, power up the G1, job done!