Android 3.0 targets devices with larger screen sizes and it's is expected to be the OS of choice for the flood of touch based tablets due during this year.You can read about its complete set of new options in Android 3.0 Platform Highlights. The overall conclusion seems to be that the OS is better suited to the larger format and has some welcome enterprise features.
You can see it in action in the Google promo video:
A UI framework larger screens
A range of new UI components, new themes, widgets and notifications, drag and drop and so on.
Improved 2D and 3D graphics
A new property-based animation framework and a built-in GL renderer for 2D hardware-acceleration. For 3D scenes, developers take advantage of a new 3D graphics engine called Renderscript.
New types of connectivity
New APIs for Bluetooth A2DP and HSP let applications offer audio streaming and headset control. Support for Bluetooth insecure socket connection lets applications connect to simple devices that may not have a user interface.
Enhancements for enterprise
New administrative policies, such as for encrypted storage and password expiration, help enterprise administrators manage devices more effectively.
The UI Builder also now supports the animation features and better drag-and-drop support. It also works better on multi-core processors.
Thankfully it claims to be backwards compatible and applications targeted at earlier version of Android should run perfectly and even take part in some of the new user interface features.
The SDK includes a preliminary 3.0 image which can be used with the emulator to test applications. Of course you can't submit new 3.0 apps to the Android Market until the final SDK is released.