Apple's annual developer conference in San Francisco.
Video Shows Differences Between Mavericks and Mountain Lion Multiple Monitor Support
In Mountain Lion, there are issues with using full-screen apps on setups with multiple monitors. Putting an app into full-screen mode disables the rest of the monitors. In Mavericks, apps can be set to full screen on individual monitors and moved around as necessary.
However, as this video shows (via 9to5Mac), there are some changes in Mavericks that multiple monitor users may not like. For instance, application windows cannot be used across multiple monitors anymore -- windows can only be used on one monitor at a time. Also, spaces are only switched individually, which could affect the workflow of some users.
It appears that, in OS X Mavericks, users can switch between both the Mountain Lion multiple monitor setup and the new multiple monitor support in Mavericks as needed. As always, it's worth noting that this is a beta and things could change before the final version of OS X Mavericks is released in the Fall.