I am pretty pleased with my T-Mobile G1 Google Android device (see my review) and in a couple of months I understand I will be able to unlock it and use it with my AT&T SIM card or overseas too. If you want an unlocked G1 now, according to Engadget you can buy one for just $424 right now. The G1 is US$399 and then you have to register as a developer and pay the US$25 fee to get access to the purchase option from T-Mobile USA.
This new developer version is the black model (the only one I recommend you buy because of the visible keyboard) and also looks to come with a custom designed back cover that looks pretty slick. I think this is a great deal on the G1, since mine cost me just about $100 less since I was an existing customer that recently upgraded my phone and this would have been the better deal because no contract extension would have been required and I could have kept my old (no grandfathered) total internet plan and saved myself $5/month too. The devices is shown as coming soon and I am not sure when it will actually ship to buyers.