Here are the release notes for 10.6.6:
The Mac OS X 10.6.6 Update is recommended for all users running Mac OS X Snow Leopard and includes general operating system fixes that enhance the stability, compatibility, and security of your Mac. It also includes the Mac App Store, the best way to discover and buy new apps for your Mac. The Mac App Store, a new application you’ll find in the Dock, includes the following features:
- Discover Mac apps: Browse featured apps, top charts, and categories, or search for something specific. Read detailed app descriptions and user reviews, and flip through screen shots.
- Buy and install: Easily purchase apps with your iTunes account. Apps install in one step and are quickly available from the Dock.
- App updates: The Mac App Store keeps track of all your purchased apps and notifies you when free updates are available.
After reboot, the Mac App Store will appear in the Dock. We’re currently exploring it, and will have some details up soon!