I'm working on recurrent event today (well, I'm sick, so it's more something like: I'll try to work on recurrence :/).
I wonder what kind of recurrence is needed? It seems obvious that we need common: every n days/week/month/year stuff, and be able to limit the recurrence to a number of ocurence or an end date... but what else? do you need something else?
Personnaly, I'd like to add the "period" concept: for instance, you'll define a period called "hollydays" and define an event as "every monday except during my hollydays".
The main problem imho, will be syncing it with external device (or other PIM), vCal doesn't contains such things, pocketpc doesn't (and I think palm doesn't), so syncing will implies some kind of lost.
So the question is: is it worth? The fact is that you'll get at least what a pocket oulook PDA can do, is more worthful?