[whpalmer4]

There's a script floating around on the forums that will make an iCal event (not a To Do, which is what the OmniFocus code does) out of a selected item. You could bring up a context mode view, grouped by due, select all the items that have due dates assigned (click on the one at one end of the list, shift click on the one at the other end of the list), and run that script. It won't however be a true sync; any changes you make in iCal will not be propagated back to OmniFocus, and I suspect that if you did this multiple times you might end up with duplication in iCal, so you'd only want to do it with items that newly had due dates assigned. Changing the due date on an item in OmniFocus will force you to go change it in iCal as well. I suppose you could have a special calendar in iCal that you used only for this, and just empty it out each time before shoveling over all of the OmniFocus items. It wouldn't be too hard to have the script make To Do entries instead of calendar events, if that's what you wanted.

As for the business about whether or not this might follow David Allen's teachings, do not forget that OmniFocus is not designed for use only by strictly-by-the-book GTD practitioners. The iCal sync support was put in to allow people (irrespective of their personal productivity belief system) to get their OmniFocus data onto their mobile device in some fashion prior to the advent of the iPhone/iPod touch sync support. It doesn't put any events in the calendar, only actions in the to do list, so even the most strict interpretations of DA's writings shouldn't be in conflict.

It would seem like this would be a straight-forward modification of the existing code to do exactly what you propose, but there are some issues to be thought out that arise. What is supposed to happen if you change or remove a due date in iCal? What if you delete the item in iCal, or move it to a different calendar, possibly one not being synced with OmniFocus?