With Nepomuk and Zeitgeist complementing each other nicely, the KDE developers are looking to leverage both projects so that the KDE desktop can then effectively provide answers for commands like "Give me most listened to songs by Michael Jackson." Alternatively, "give me all recent contacts tagged with work that I contacted from home."
It will take some months before you will see Zeitgeist appear in KDE, but all of the details surrounding their plans and the collaboration between developers can be found in this blog post. Meanwhile, on the GNOME side, Zeitgeist will be properly making its debut with GNOME 3.0 in March. One of the ways the engine is used within this release is via the GNOME Activity Journal.