From the Chronicle of Higher Education. Here is the first paragraph from their "About the Data" section:

Salary data are collected annually by the American Association of University Professors. The most recent data are for the 2011-12 academic year. Salaries are reported in thousands of dollars and rounded to the nearest hundred. For consistency, they are adjusted for a nine-month work year. The figures reflect the earnings of full-time members of each institution’s instructional staff, except those in medical schools.

A word to the wise: always be cautious when you hear the words "average" and "salary" in the same sentence. I leave it to the reader to consider whether these salaries are consistent with the idea of a bubble in higher education.