I stick with the cannonical GTD approach: I have a subset of certain contexts (i.e. "Errands:Hardscape") that I sync with iCal (although I am a BusyCal user). This way, tasks that must be done on that day are on my calendar. A majority of the time, if it needs to be done that day, I can also assign a time and an alarm to the task (in BusyCal). I evaluate these items during my weekly review.
A disadvantage of this is that these tasks assigned to the subcontext are not sorted with all of the other tasks in the "meta"-context (in this case, the context "Errands"). If I need to modify the task, I need to remember it is "hardscaped".
I have often though that a checkbox in the inspector would be better than a subcontext. This way, if the task needs to be in my calendar, that fact becomes a property of the task, not a context to force the property. I'm not sure how deep you could drive the info into a local calendar or CalDAV. It would be awesome if calendar properties could be applied in the inspector (i.e. alarm, time; maybe even specific calendar in iCal (you could assign the task to the "delegate" calendar in iCal or the "Hardscape" calendar in iCal)).
The other half of this is what happens to the task on the calendar. I send my "hardscaped" tasks to specific calendars. When reviewing my calendar, I delete all the tasks in that calendar (if I need to keep the task on my calendar for some reason, I move it to another calendar). This way, whatever changes I made via Omnifocus are imported to it's proper place without worry of duplicate tasks, etc. Of course, if I could know that I could reliably check a task as "done" in BusyCal, and a sync would modify it correctly in OmniFocus, I would do that. But Sync Services is fragile and I choose the more labor intensive management to avoid the frustration of having to unwind the damage Sync Services had wrought. Obviously, Omni would love Sync Services to be more reliable also. I have often wondered if Omni would benefit from forming a relationship with calendar suppliers (Google, BusyCal) and develop a sync program that runs out of system preferences.
To sum up, I would like to see calendar syncing being driven as a property in the inspector, and would like to see Omni bypass Apple's sync (and attending issues) by developing Omni's own sync service.
Thank you for soliciting feedback on this issue.