Riding The Nexus One Wave, Google Releases The Android 2.1 SDK
As noted on the Android Developers Blog, the team is releasing it before most devices have it so that developers can play around with and build for the new features introduced in 2.1. Though Google calls 2.1 a “minor platform release” over Android 2.0, there are a number of new elements such as voice recognition, live wallpapers, a new launcher, more home screens, and some WebKit changes. Those who have ported it over to the Droid note that the new OS is also faster.
Google also notes that there is a new USB manager available through the SDK Manager that supports the Nexus One. This may or may not be related to the new services that it seems like Google wants to include with the device, such as a new docking station for backing up your data.
You can find the new SDK here.