Apple's annual developer conference in San Francisco.
OS X 10.9 Activity Ramping Up as Speculation on Developer Preview Release Increases
Hits on macrumors.com from systems identifying themselves as running OS X 10.9 in January 2013
Apple needs to test new platforms with a wide range of websites, and server logs can be examined to see which operating systems and browsers are in use, tipping off sites when a new OS appears. While such data can be easily faked in browsers, the trends currently being observed are consistent with past releases and previous examinations have shown OS X 10.9 hits coming from IP addresses associated with Apple.
Growth in hits on macrumors.com from systems identifying themselves as running OS X 10.9
Apple has recently been launching new versions of OS X on an annual basis, with OS X 10.8 Mountain Lion having launched in July of last year, a year after 10.7 Lion in July 2011. Apple released the first developer preview of OS X Mountain Lion in mid-February of last year, so it seems reasonable that the company could be looking at similar timing for OS X 10.9.
It has been rumored that OS X 10.9 will bring Siri to Macs for the first time, building on the dictation feature added to Mountain Lion. Rumors have also suggested integration with Apple's Maps, although the company has experienced significant criticisms with its maps app on iOS.