Olive is a set of add-on libraries for the Mono core that bring some of the new .NET 3.0 APIs (Windows Presentation Foundation (WPF), Windows Communication Foundation (WCF), Windows Workflow Foundation (WF)) to Mono.
Olive is unsupported, should be considered as experimental software, and since it implements a shifting API there are no guarantees of any kind about the stability of the API. WPF is an enormous API that will take considerable time to implement.
At this point (Feb. 2008) you should not consider it usable, additionally we have no plans to work on it anytime soon. WPF will likely use a separate codebase than Moonlight does for its gui features. 
"It is probably a long ways off (many years), if ever. I don't think we've decided if we will be doing WPF."
"However, the rest of 3.0 (WCF, Infocard, and WF) already have partial implementations for Mono, and will be completed, though I am unaware of the timeframe."