There has been other commentary from Matt Cutts and Shuman Ghosemajumder (who was a Sloanie, btw). The latter contains a key admission, something not explicitly stated by Google before (at least from what I've heard and seen):
"So what is our overall "click fraud rate"? As noted in the diagram in the story, it is virtually impossible to know the intent of every click. However, we can do a very effective job using statistical techniques to detect potentially malicious behavior, and the total number of invalid clicks we detect – whether for suspected malicious or non-malicious intent – is in the single digit percentages. So third-party estimates which say that click fraud is 15% or higher appear to clearly be substantial exaggerations."
While this statement is plausible on the surface, it still underscores that the actual amount of click fraud is still unknown, and will continue to be when the determination is made using logs, clickstreams, etc.