Ambient occlusion is enabled via application profiles in the NVIDIA Control Panel. Application profiles account for the different rendering needs of each game, and enable ambient occlusion accordingly.
Currently the following 22 titles are enabled with ambient occlusion profiles:
Assassin's Creed
BioShock
Call of Duty 4
Call of Duty 5
Call Of Juarez
Company Of Heroes
Counter-Strike Source
Dead Space
Devil May Cry 4
F.E.A.R. 2
Fallout 3
Farcry 2
Half Life 2 series
Left 4 Dead
Lost Planet: Colonies
Lost Planet: Extreme Condition
Mirror's Edge
Portal
Team Fortress 2
Unreal Tournament 3
World In Conflict
World of Warcraft