Mark Thoma at Economist's View is doing some work in the area. In a recent piece, he shows the grade book from a 1949 principles level history class. It breaks down to a class GPA of 2.22, with only about 9% getting A's.
Thoma's own work indicates that grade inflation is more pronounced among assistant professors, teaching assistants, and instructors, and not as much of a problem among associate or full professors. This isn't surprising - those lower on the academic food chain have an incentive to game the system for higher evaluations, and grade inflation is one means to that end. He provides some stats:
Here's one measure across faculty rank, %A (other measures also show this). We have three levels of courses, level 1 is principles and level 3 is upper division. Here are the numbers:Click here for the full posting.
Full professors (%A in Levels 1, 2, 3)For comparison, the grade distribution from 1949 given above has a %A of 9% for a level 1 class (13/140).
26% 31% 35%
Assistant professors (%A in Levels 1, 2, 3)
30% 45% 42%
Adjunct professors (%A in Levels 1, 2, 3)
38% 50% 42%
Thoma also provides a link to gradeinflation. com, which has a lot of data on trends in grades for a wide variety of schools over time. A casual look at the numbers shows that the typical school's GPA has gone up by around 0.2 since from 1991 to 2002. Looking at the numbers, it also seems like that the inflation is less for lower-tier public schools (particularly non-flagship ones), but I can't back that up with hard analysis.
As is often the case on issues like this, the comments following Thoma's piece are almost as good as the piece.