A few minutes ago Apple released OS X 10.6.6 with access to the Mac App Store in Software Update. It is available now as a free 114 MB download for Snow Leopard users. As noted by Macworld, the Mac App Store already includes some apps from Apple such as the iLife '11 suite with standalone apps sold at $15 each. Sadly, there's no iWork '11 in the Mac App Store -- the iWork '09 apps are sold at $20 each.
Aperture and Apple Remote Desktop are also on sale in the Mac App Store, both available at $79.99.
Full changelog and first screenshots of the Mac App Store below.
Update: here are the direct links to the OS X 10.6.6. update.
Update #2: And here's everything you need to know about the Mac App Store.
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.
To learn more about the Mac App Store, visit: http://www.apple.com/mac/app-store.
For detailed information on this update, please visit this website: http://support.apple.com/kb/HT4459.
For information on the security content of this update, please visit: http://support.apple.com/kb/HT1222.