Today, we are releasing the SDK component for Android OS 2.1, so that developers can take advantage of the new features introduced in Android OS 2.1. Please read the Android 2.1 release notes for more details. You can download the Android 2.1 component through the SDK Manager.

In addition to the new SDK, a new USB driver that supports Nexus One is also available today through the SDK Manager. The USB driver page contains more information.

Source: http://android-developers.blogspot.com/2010/01/android-21-sdk.html