Apple today seeded the first beta of OS X 10.9.1 to developers for testing. Apple states that developers should focus on email (because Mail.app under Mavericks is basically broken), graphics drivers, and VoiceOver. There are no known issues with this build, meaning that 10.9.1 could be released relatively soon — likely before the Holidays. It’s also possible that 10.9.1 will be released alongside the updated Mac Pro.
This would be the second update for Mavericks since its release. The first came as a special update that was designed to fix Gmail under Mavericks. The update helped some issues, but failed to fix everything.