Zeitgeist 0.3 is still an unstable release (Zeitgeist 0.4 will be the first stable version) and carries a host of changes. In fact, the Zeitgeist engine and D-Bus API have been completely reworked since Zeitgeist 0.2. This new API is much better, but early Zeitgeist adopters will already need to update their applications against this latest interface. Besides overhauling the engine and API, Zeitgeist 0.3 features a public API for Python, changed Ontology from XESAM to Nepomuk, the Storm and Querymancer back-ends have been dropped, support has been added for event payloads, and an extension API for the core engine has been introduced.
There's plenty of other work going on with GNOME's Zeitgeist too, which is talked about in the 0.3 release announcement.