The problem is that any importer would need to deal with an astonishing number of variations in the format of the data, computers are netoriusly bad at adjusting to that sort of data. At this point I have not seen any transltor that even attempt to do this, and it would take a lot of work.
The best of all worlds is that the organizer would use any of the programs that use the ical format and publish the file to a server that everyone could get access to. The calendar/sunbird project over at Mozilla.org would be a free solution for the Windows/linux crowd.. and of course it is free. It is still a pre-release piece of software, but useable.