As mentioned earlier here's a link to a working example of a repeat event calendar picker. It isn't pretty, but it helps formalize the thinking Joe and I did.
This doesn't actually "make" a valid repeat event string in say iCal but simply exercises a UI for being able to pick calendar events.
http://makerlab.com/projects/20081013_Calagator_Repeat_EventsI threw it on makerlab for convenience but obviously it is open source / free / volunteer just to be totally clear.... as is everything I personally contribute to...