From the Windows Vista blog:

Designed to be user-friendly, the new packaging is a small, hard, plastic container that’s designed to protect the software inside for life-long use. It provides a convenient and attractive place for you to permanently store both discs and documentation. The new design will provide the strength, dimensional stability and impact resistance required when packaging software today. Our plan is to extend this packaging style to other Microsoft products after the launch of Windows Vista and 2007 Office system.

A copy of the now-popular MyBook drive casing?