This was spotted by Mary-Jo Foley who has received excited beta tester feedback.
The three main features available in the beta are as follows:
Active Storage Platform: This pre-release package enables Windows to restrict access to portable devices (such as a USB flash dongles) via a certificate or password authentication
Image Mastering API update for Blu-Ray media: This feature enables Windows to do master style optical burning on Blu-Ray media
Smart Card Driver: Provides support for new form factors, such as ICCD/CCID
Unlike Service Packs which generally bundle a set of fixes and security patches, feature packs are intended to deliver additional functionality between Windows releases.
Microsoft has previously released the 'Wireless Vista Feature Pack' in July, available here.