Apple has sent messages to Mac developers asking them to begin submitting apps that are optimized for the latest update of OS X, codenamed Mavericks. The announcement read, "Make sure your app takes advantage of the great new features in OS X Mavericks when the world’s most advanced desktop operating system becomes available to millions of customers later this fall. Download OS X Mavericks GM seed and Xcode 5.0.1 GM seed, now available on the Mac Dev Center. Build your apps with these latest seeds, then test and submit them to the Mac App Store."
These sorts of announcements usually signify that Apple is about the release a product to the general public. Apple has a press event scheduled for October 22, and some are speculating that it could announce availability of Mavericks then.