[X-Cubed]

iCalendar is a pretty nice format to use and well supported, but the trouble that you then need a website to host the iCalendar file. It adds more yak-shaving to the process.

[jka]

Yep, true; how you get the iCalendar file to the user could affect the requirements and experience a lot.

Web could work well for pull-based retrieval, and email's probably a good push option when you have a defined set of recipients.