Google’s biggest threat is an encyclopaedia
With increasing scrutiny over the accuracy of the content, wikipedia is an increasingly trustworthy source of information.
Properly monetised (with even wikipedia adwords??) they may even be able to launch further afield…wikipedia films, wikipedia sport (like mark Cuban’s wiki).
In the same way adwords rewards good keyword selection, [tag]wikipedia[/tag] subject-specific ads would surely trump [tag]Google[/tag] for ROI.