[Brian]

Personally, I use iCal for the items that are fixed in time - repeating meetings I need to schedule around, or singleton events that nevertheless occur at a specific time/date.

OmniFocus holds my big stack of items that need to get done on the macro level, but don't need to be scheduled at the micro level. (I use the 'Due' column in OF a lot more often than I use the Start column.)

iCal 'pushes' events to me; I 'pull' actions from OmniFocus when I'm between those events and am not sure what to work on next.

In general, iCal *or* OmniFocus will notify me of something, but not both.

(We've heard from several users that they'd like some sort of status report generation in OmniFocus; I think that would probably also help folks feel comfortable with this sort of divided workflow.)