Horde_Form_Type_nag_recurrence should use $vars instead of 
Util::getFormData() and Nag_Recurrence instead of 
Horde_Date_Recurrance. And we probably have to use different constant 
names to avoid collisions when showing tasks in Kronolith. I just see 
you already did this, but you don't use them everywhere yet.

The varrenderers won't work the way you implemented them though. 
Either you need different form fields for the different recurrence 
settings, or you have to implement the complete widget in a single 
cell, like any other form field.

And you shouldn't create new tasks when completing one recurrence 
instance. Instead, you should store the completion of a single 
recurrence, similar to how exceptions are stored at the moment. And of 
course we should (at some point, not necessarily from the start) add 
the ability to actually create exceptions.

But beside that, it looks very promising, nice work!

A draft for implementing recurrence in Nag. I did only implement it 
for the kolab driver so far. Does this go into the right direction?

Support recurrence in tasks.  Should just be a matter of extending Nag 
to use the recurrence portion of the iCalendar spec.