Evidence of Apple testing OS X 10.9 first surfaced late last year, when AppleInsider tracked a handful of visits from machines running the next-generation operating system in the month of October. But while October visits from OS X 10.9 were around three dozen total, the number swelled into the thousands in the just-concluded month of January.
We have also confirmed some traffic from Mac OS X 10.9 on iClarified. Apple announced OS X Mountain Lion 10.8 on February 16th of last year so it's very possible that we could see an announcement for 10.9 in the next few weeks.
Both Lion and Mountain Lion were launched in the month of July.