Stuart Rojstaczer (Professor Emeritus, Duke University; owner, www.gradeinflation.com) & Christopher Healy (Professor of Computer Science, Furman University) have posted Grading in American Colleges and Universities on Teachers College Record: The Voice of Scholarship in Education.  Here is the abstract:

Here we report on historical and recent grading patterns at American four-year colleges and universities. Records of average grades show that since the 1960s, grading has evolved in an ad hoc way into identifiable patterns at the national level. The mean grade point average of a school is highly dependent on the average quality of its student body and whether it is public or private. Relative to other schools, public-commuter and engineering schools grade harshly. Superimposed on these trends is a nationwide rise in grades over time of roughly 0.1 change in GPA per decade. These trends may help explain why private school students are disproportionately represented in Ph.D. study in science and engineering and why they tend to dominate admission into the most prestigious professional schools. They also may help explain why undergraduate students are increasingly disengaged from learning and why the US has difficulty filling its employment needs in engineering and technology.

• Chronicle of Higher Education, Grade Inflation Pumps Up GPA's by 0.1 per Decade, Scholars Find
• USA Today, Analysis Documents College 'Grade Inflation' Over Decades
• Inside Higher Ed, Grades on the Rise:

Grades awarded to U.S. undergraduates have risen substantially in the last few decades, and grade inflation has become particularly pronounced at selective and private colleges, a new analysis of data on grading practices has found. … [They] illustrate that grade point averages have risen nationally throughout most of the last five decades. The study also indicates that the mean G.P.A. at an institution is “highly dependent” upon the quality of its students and whether it is public or private.

There’s no doubt we are grading easier,” said Rojstaczer, the founder of GradeInflation.com, where he’s built a database of grades at a range of four-year institutions since 2003. The findings are based on historical data dating back at least 15 years at more than 80 colleges and universities, and contemporary data from more than 160 institutions with enrollments totaling more than 2,000,000.

Since the 1960s, the national mean G.P.A. at the institutions from which he’s collected grades has risen by about 0.1 each decade – other than in the 1970s, when G.P.A.s stagnated or fell slightly. In the 1950s, according to Rojstaczer’s data, the mean G.P.A. at U.S. colleges and universities was 2.52. By 2006-07, it was 3.11.