If you do not want to have any contractual agreement with T-Mobile but still you want to put your hands on a shiny G1 phone, the solution is here. Actually, the key to this problem is offered by Google.
Anyway, Google is now selling a version of the G1 that has neither SIM lock nor bootloader lock. Basically, you can add any SIM card and it will work perfectly. The unlocked Google phone goes by the name of Android Dev 1.
The handset is available to registered Google Android Developers for about $400 USD. You might want to add an extra $25 if you want to become an official Android developer.
The handset can be ordered from these countries: USA, UK, Germany, Japan, India, Canada, France, Taiwan, Spain, Australia, Singapore, Switzerland, Netherlands, Austria, Sweden, Finland, Poland, and Hungary.