Two scenarios here:
1) - user receives meeting invitation
   - server adds event from it to users calendar automatically
   - user selects invitation in the mailer view
     result is that the invitation reads "Not found in any calendar", while,
     if a Refresh is done in the corresponding calendar, the invitation
     will find it

2) - update for a previous meeting invitation is received
   - server automatically propagates changes to user's calendar, like
     making from single instance a recurring event
   - if user accepts change in evolution then he/she can receive a "Conflict"
     response, which means the local version of the calendar event is too
     old and does not correspond to current state on the server

For the first case will work to check server for updates if asked for a particular event (like in ECalBackend::get_object). For the second case may check for updates on the given object in ECalBackend::receive_objects and possibly also in ECalBackend::send_objects. Both only if the backend is online.