[Grail]

Not entirely related to OmniFocus, but what about simply putting a recurring appointment into iCal? Then when you can't attend the "Viola Practice" appointment, just delete that appointment.

This means that you have the time already blocked out for that task, and there's no messing about with plonking other appointments into that time slot without consciously making the decision to move Viola practice back or forward in that slot.

If you're doing "Viola Practice" every second day, or three times a week, or whatever - you can just move the appointment around in iCal.

I have the same thing in my calendar for things as trivial as "Bed Time!" and "Guitar Practice" - I'm a gamer, and I need my Palm to nag me about bed time otherwise I'll be thinking, "just one more level/quest..." then it's 3am. And my guitar sits next to my computer untouched.