OS X 10.9 details get spilled ahead of WWDC 2013.
The WWDC 2013 is mostly about software. Apple will show developers the upcoming OS X and iOS version. OS X 10.9 aka Cabernet details have leaked today on 9to5Mac. Their sources say OS X 10.9 is focusing on features for power users. It will not contain a major UI overhaul.
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New features will be found in the Finder, Safari, multi-monitor handling and lots of other smaller things. If Siri is part of OS X 10.9 is not clear.
Overall OS X 10.9 sounds not very exciting. The more interesting topic at the WWDC 2013 is definitely iOS 7.