When the dollar appreciates from its present level relative to the Swiss Franc by ~250%. Before this happens, it's likely that oil will be priced in Euro's rather than dollars. Of course it is going to come down from here in the near term, but not to $40!
there is plenty of undiscovered oil, but the rate of discovery keeps dropping while consumption keeps increasing and the known oil reserves are being depleted. there will NEVER be enough oil (meaning the price will keep rising) until new technology is developed which is not going to happen for 10-20 more years.
CIBC report puts crude at $225 a barrel by 2012 http://cosmos.bcst.yahoo.com/up/player/popup/?rn=289004&cl=7541194&src=finance&ch=4043681
If someone comes out with a form of energy that replaces crude/gas and is widely renewable or available, expect crude to hit $40 again. Barring that miracle, never.