Today's Mac App Store update is available to all OS X Snow Leopard users and can be downloaded through the Mac App Store's software update mechanism.
Before becoming available for public release, the OS X Snow Leopard Mac App Store compatibility update was made available to developers on January 20 for testing purposes.
OS X Snow Leopard (aka OS X 10.6) was first released in 2009. As software that has been discontinued and is run primarily on older machines, Snow Leopard updates are few and far between. The last significant Snow Leopard update, aside from security fixes, was introduced in 2011 ahead of the release of OS X Lion.
Apple's Mac App Store first launched in 2011 as part of the OS X Snow Leopard 10.6.6 update.