Yeah, I hear about that from time to time.  The mother of a good friend of mine is some sort of guru in special education (she's been getting invited to Cyprus every year lately to consult with their government's version of the Department of Education, among other things).

She has a fat text book that she updates (and sometimes completely rewrites) every 2-3 years, which is at least a $50 book and sells a very predictable but fairly low number (just not that many people getting specialized degrees in Special Education).

When the last version went out in mid 2010, someone pointed out to her that there was a "warez" version of it readily available less than two weeks later.  Sales on the new version are nearly 20% off the expected numbers so far.

Who knows what will happen to the sales of this version through the rest of its life cycle will be, but I'd call that a very distinct and measurable impact from piracy of one specific volume.  Heck, it might even turn into a study in "book piracy's impact on education," if she decides its not worth the time and effort to continue supporting her work.