Apple has asked developers to begin submitting OS X Mavericks-optimized apps. On Apple's official News and Announcements page for developers, the company said:
Submit your OS X Mavericks apps today.
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.
The timing of the request correlates to Apple's announcement today that it is holding another product launch event on October 22. Though the latest Mac Pro and MacBook Pros are expected to be announced (along with new iPads), it's unlikely that OS X Mavericks will ship that day. However, the fact that Apple is requesting devs get their Mavericks apps ready suggests the new OS could launch in the days following the event.
Apple OS X Mavericks