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I have everything set to maximum. I have tried AO off, performance, and quality settings and I see zero difference. I have tested multiple times and areas and no difference.
AO on: http://i.imgur.com/sdAZL.jpg
AO off: http://i.imgur.com/e6eLZ.jpg
After playing with the settings more, ambient occlusion does not work with Reflections turned to All and Supersampling must be turned off. However Supersampling increases draw distance on grass and other stuff for some reason, with minimal increase in picture quality. So you have to choose between ambient occlusion or draw distance.
Also ambient occlusion will not show up in screenshots because it flickers off while it's creating the file.