People love their mobile phones because they can stay in touch wherever they are. That means not just talking, but e-mailing, texting, microblogging, and so on. So, in addition to search by voice and voice shortcuts like "Navigate to", we included a voice-enabled keyboard in Android 2.1, which makes it even easier to stay connected. Now you can dictate your message instead of typing it. Just tap the new microphone button on the keyboard, and you can speak just about anywhere you would normally type.

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