I have been working on providing a good e-cal wrappers in evolution-sharp as the default GAPI stuff don't work for most of the required-APIs because of the intensive use of ical APIs and data definitions.
What I have learnt from this short experience is that any application can be extended to provide good mono API wrappers that can be used to extend these application or write new cool stuffs using them.
Also, I should request for a big hand for GAPI, the provider of these wrappers. Though there are pits and falls, still GAPI rocks!!
Evolution has support for plugins written in Mono. This is a good welcoming step taken by the Evolution team. I would request the evolution team to actively maintain and update evolution-sharp as well, because, people will love to have bindings for the cool features that Evolution has.
(I can be one of your active contributors as well, if you guys like me. ;) )
I heard OpenOffice also supports Mono, but, don't have any exact data on what sort of support they provide. (for running VBA scripts?)