It seems T-Mobile has taken a different stance on G1 activation than Apple did with the iPhone. Apparently, all you need to do is slip in a T-Mobile-activated SIM card and run through the phone’s setup process. Once that’s done, you can put your new SIM back in for full Android access expect for network access or apparently, keep the old one active for full network access except for voice calls. This is still developing so stay tuned as more details float in.
LoveForBiz via Eng
T-Mobile has always been really good with their network usage. As long as you have an Active T-Mobile SIM card you can jump on their network with practically any device, and their technical support staff will help you too!
You have to have a data plan as of now to activate the G1 phone..