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The icalendar and xcalendar import filters currently warn you if they encounter "unsupported" or "unrecognized" tags (such as vevent). It would be better if these were stored (as regular icalendar content lines) with the task, and on export written back out. This is easier for icalendar than for xcalendar (unless I switch to DOM). Anyway, that would make it a lot easier to share an .ics file between multiple applications, since it wouldn't lose any data.
Fixed in trunk (iCalendar)