It's not just for Microsoft stores anymore. The 4GB Xbox 360 with Kinect and a two-year contract will soon be available in US Best Buy Stores and select GameStop locations later in June. You'll have to contact your local store to find out if it's participating in this expanded "pilot program."
Reminder: the math for the contract bundle is roughly $460 over the next two years. The 4GB Xbox 360 Kinect bundle has a suggested retail price of $299, with Xbox Live Gold subscriptions costing about $50 a year, for a upfront total of $399 if you bought now.