Apple first confirmed the existence of OS X Mountain Lion on February 16, 2012, offering a preview of some of the more compelling features. After a more extensive demonstration at the Worldwide Developer’s Conference on June 11, Mountain Lion became available for download on July 25, precisely on schedule, for $19.99.
So if Apple is really planning on annual OS X upgrades, it would make sense to see the first glimmers of an OS 10.9 feature set this coming week. But that assumes Apple is really keeping to that schedule, and that there are enough finished features to warrant such a presentation.
It’s not as if Apple would consider holding a special media event for a Mac OS upgrade. There will probably be one for the next generation Mac Pro, though it might just be a part of the agenda at the next WWDC. That would appear to make sense, since a professional Mac workstation would be well suited to the developer community.