The Google Nexus One has dropped its upgrade price for T-Mobile customers. Instead of paying $379, you'll only have to pay $279 (and if you paid $379, you'll get a $100 rebate).
New T-Mobile customers can get the Nexus One for $179 if they sign a 2-year contract. Speaking of the Google Nexus One running Android, it's coming to Verizon and Vodafone this spring (Spring 2010). If you're on Verizon and you can't wait, then you always have the option of getting the Motorola Droid or the HTC Droid Eris.