If I understand the original post with the Start/Due dates in iCal, I believe the intent is to have my iCal show a task as an event within iCal. If that is true sign me up. If not, read on!
The second route is another trick done by Omni to make up for deficiencies in the Apple API support for notifications an iOS release or two ago. Because there was no provision for an application to send a notification to the user without assuming that there was ubiquitous internet connectivity, someone at Omni had the clever idea of populating a calendar on the device whenever a sync is done with appointments of 0 minutes duration and letting the calendar app (which could do those notifications) handle the dirty work of notifying the user that something was about to be due. This tactic uses actual calendar events, not to dos, and so some people have used this as a way to see their upcoming due items on a calendar. However, the notification is the same amount of lead time for every action (one size fits all!) and nothing whatsoever is done with start dates. It also is no longer needed for devices running recent iOS releases. Finally, it is only populated for 10 days or 2 weeks into the future, so even if you are okay with the other constraints, it may not show as much of a view as you want.
I have tasks as taking only 5 minutes in my forecast and/or within iCal. However, some tasks are not realistic to complete in 5 minutes or less. Take wrapping gifts for charity task(sticking with Christmas theme), I would like to estimate the effort to take 4-6 hours. My hope is for OF to forecast that out enough based on a start or due timestamp. My preference is that I know when it needs done, so back the estimate from there and show/force my hand to complete the task.
Hope that made sense and didn't complicate matters.
The GTD (and OmniFocus) workflow boils down to this:
1) Look at calendar. If there is something to do right now, do it. Go to 1.
2) Pick something to do off your lists. Do it. Go to 1.
I remember someone posting a reference to a web app somewhere that created a schedule for you based on all of the things you need to get done. We discussed it a little bit on the forum here, but I don't remember enough keywords to find it.