updated 01:25 pm EST, Tue November 6, 2012
OS likely in extremely early stages of development
Computers running OS X 10.9 are already showing up in web tracking, notes 9to5Mac. The site says it has picked up at least four IPs running the OS during the last month, and that the addresses trace back to Apple's corporate network and the wider Bay Area in California. At least two of them are said to be using a screen resolution consistent with a 15-inch Retina MacBook Pro.
Although the data could be faked, Apple engineers have been known to take early versions of an OS online for testing. The company would also likely need a semi-functional version of 10.9 by now, since OS X development has switched to an annual track. 10.9 will probably be shown in June at WWDC, if not earlier.