Position depends on how interested the visitors are.
If they are really,really interested, they won;t go away on an ad until they're done (in that case the end of the doc maked sense.
If you have visitors that are only slightly interested or are dropping in and actually looking for something else, they might use an ad to get to something instead of pressing the back button.
-> above the fold and very visible works better.
In other words: know your visitors! (as hard as that is)