hm. it seems like the mozilla calendar component does a reasonably good job at the iCalendar client aspect. The protocol may be crap, but I'm not sure what it would mean to be complete beyond what it already does for me -- that is, keep track of appointments. And the Gecko framework is a reasonably nice UI, from my point of view.

Not sure what you would want that's a lot different -- is there something fundamentally wrong with the Moz calendar approach?