Yes you can now buy the Nexus one phone, its officially released at http://www.google.com/phone. Nexus One is not only be available on T-Mobile, but it will also be available on Verizon Wireless later 2010. If you want to buy the Nexus One phone unlocked, it will cost you $529. If you want to buy it with T-Mobile it will cost $179 with a two-year contract.
Below is the screenshot of the official Google Site For Nexus One