Cartels rarely work in the long run because they can’t be policed. OPEC is no different (via SFGate):
Hours after suffering a rare setback in a negotiating session at OPEC’s headquarters, Saudi Arabian officials assured world markets on Wednesday that they would ignore the wishes of other cartel members and continue to pump plenty of oil.
Trying to keep prices high in the face of falling demand would seem to be a losing proposition. The Saudis seem to get that even if the rest of OPEC doesn’t.
Update: Mish Medlock has a longer and probably better written explanation of why OPEC is rarely effective at controlling oil prices.