This is not usually cost effective but they just wanted iCalendar files and they were very polite about the request.
Most times this would not be worth mentioning, but they had a novel approach -- they wanted to obsolete their legacy Meeting Maker server ASAP and move on so they took their data from Meeting Maker into Apple iCal before planning on putting it into Zimbra.
The good news: Seems to have worked fine!
To quote their plan:
...for now it will reside in iCal until Yahoo gets done with the updates that
(we) asked for. Then we will be able to sync iCal to Zimbra.
I really have to give them points for creativity.