They had nothing to announce. Technology is not always going to run on a set schedule. The rumors are that Apple is behind on development on iOS that they have pulled people off OSX development to finish up.
Mountain Lion also represents a dramatic speedup in the pace of OS X updates: Apple says it’ll be issuing yearly updates from now on. That’s partially to match the breakneck pace of iOS development, but also to capitalize on the growing popularity of the Mac in general — Mac sales have outgrown the general PC market for something like six years now, and Apple says it’s investing heavily in the platform to build on that trend.