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.
Android 2.1 is a minor platform release deployable to Android-powered handsets starting in January 2010. This release includes new API changes and bug fixes.
Google also notes that there is a new USB manager available through the SDK Manager that supports the Nexus One.
You can find the new SDK here.