Media event set for September 9, launch likely ~10 days later.
OS X Mavericks Released for All Mac Developers as Golden Master Seed
Apple has also seeded the GM build of Xcode 5.0.1 for building applications for Mavericks.
Apple is reportedly targeting a late October launch for Mavericks, with the company starting to train AppleCare staff on the upcoming operating system this week. Sources have indicated that training must be completed by October 11.
Update: Apple has also seeded developers with an iPhoto 9.4.7 update that "addresses an issue that could cause iPhoto to quit unexpectedly on launch when running OS X Mavericks GM Seed."