Previewed at WWDC, launching in the fall.
Early Signs of OS X 10.9 Unsurprisingly Showing Up in Web Logs
Hits on macrumors.com from systems identifying themselves as running OS X 10.9
A look at our own logs shows similar activity, although there has not been a noticeable uptick in hits over the past several months. Rather, we have been seeing a number of spikes in activity since at least mid-August, consistent with limited internal testing during the working week. Similar patterns were previously seen for OS X 10.8 in 2011 and OS X 10.7 in 2009/2010.
It is trivial to fake such identification strings, meaning that at least some of the data should be considered untrustworthy. But hits identified as coming from Apple's own networks carry a much greater likelihood of being legitimate, and so it seems fairly clear that Apple is indeed using systems running OS X 10.9 to browse the Internet.
A release date for OS X 10.9 is currently unknown, but Apple has stated that is moving toward a more rapid development cycle on OS X, with OS X 10.8 Mountain Lion's July 2012 launch coming almost exactly one year after the debut of OS X 10.7 Lion. Assuming a similar timeline for OS X 10.9, Apple may begin releasing developer previews in early 2013 ahead of a public launch in the middle of the year.