Apple has issued a press release announcing January 6th will be the date the Mac App Store officially opens for business.
Developers have been submitting Mac Apps for review since early November, a process that will be familiar to iOS developers by now as the two developer programs are quite similar.
Because a new build of Mac OS X may be required to run the Mac App Store, Apple could also release Mac OS X 10.6.6 on the same day or a few days earlier. According to developers, Mac OS X 10.6.6 is a very small upgrade primarily intended to support the Mac App Store; delta upgrades from 10.6.5 to 10.6.6 have reportedly been as small as 3-4MB in size.
The Mac App Store will be available through Software Update for Mac OS X Snow Leopard users only.