The implementation in Mozilla Calendar is quite simple, still very useful.

It is just a list of dates and repeating events will be omitted on those dates. In piCal this could be implemented as a textarea where you enter one date per line in some predifined format (MM/DD/YYYY) and in the table it is stored as a list in a text field. When you generates the events you just skip the dates that are in the list.

I have no idea how this relates to RFC2445. I just checked Outlook 2000 and it does not support exceptions.

Just a suggestion.

Cheers,
Marius