[jdh]

Brian is correct that there is no way to do this on the iPhone itself for the reasons he describes.

That having been said, you can sync between OmniFocus and iCal on your desktop, and then sync the respective components to your iPhone. As I've noted earlier, however, OmniFocus itself only syncs to iCal as tasks, and these do not sync to the iPhone Calendar, as it does not provide any task support.

The only way to get items that would work for your iPhone Calendar would be to sync OmniFocus items to iCal events (ie, scheduled appointments). You could use either all-day or timed events for this purpose, and even set alarms that would go off on your iPhone. Unfortunately, OmniFocus does not provide this ability itself, so you would need to use a script to do this. I'm pretty sure I saw an Applescript somewhere around these forums that can accomplish this -- you would simply run the script on your desktop and it would take your items from OmniFocus and turn them into iCal events, which would then in turn sync to your iPhone in the same way that any other calendar event does (via iTunes or MobileMe sync).

Personally, I'm not a big fan of having the majority of tasks in my calendar, as I find it clutters things up too much to use my calendar events as a task list (that's what I have OmniFocus for). I could however see the value in having certain specific milestones appear as "All-day" events, perhaps at a top-project level (ie, a scheduled project completion date being highlighted in the calendar). Selective posting of deadlines to all-day events would be a nice feature to see built-in to a future release of OmniFocus.