Beginning with a new, second OS X 10.8.3
beta build, Apple is now giving developers the option of obtaining beta OS updates through the Mac App Store's Software Update
mechanism. The move should make distribution of updates to registered testing devices more uniform, and expands the use of "developer channel" updates available through the store. Apple says that while email notification of new updates will continue as well, a new tool installed by registered developers will allow Software Update to notify users of new builds.