From Apple's Press Release :
The Mac App Store brings the revolutionary App Store experience to the Mac, so you can find great new apps, buy them using your iTunes® account, download and install them in just one step. The Mac App Store is available for Snow Leopard® users through Software Update as part of Mac OS® X v10.6.6.
“With more than 1,000 apps, the Mac App Store is off to a great start,” said Steve Jobs, Apple’s CEO. “We think users are going to love this innovative new way to discover and buy their favorite apps.”
Some notable entries on the first day are Autodesk Sketchbook Pro and the very popular iOS game Angry Birds.
CNBC quotes long time Apple watcher Tim Bajarin who says ""an important milestone for the industry that puts a nail in the coffin of shrink wrapped apps and moves them now to direct downloads."