Attrition also happens at Grad School
The Ph.D. Completion Project is a seven-year, grant-funded project that addresses the issues surrounding Ph.D. completion and attrition. In the news section the have plenty of good articles about what the rates of graduation are in different groups and what universities can do to improve their rates.
The FemaleScienceProfessor opens a room for discussion about the attrition rates or, as she calls them, the graduation rates.
In some places they actually measure those rates and make them available: Penn State Clinical Psychology Program; the UC Berkeley PhD Program in School Psychology
Some cases seem worse than other. 62% PhD student drop out rate at Flemish Universities
This is from the NSF, Summary of Workshop on Graduate Student Attrition
The Graduate School at the University of Minnesota publishes the numbers of several programs, has an amazing compilation of resources in its PhD Completion Project site. Check out the list of things that make for an Excellent Graduate Student Experience. They also have an Office for Diversity in Graduate Education. A bit out of topic, their Office of Interdisciplinary Initiatives seems like a great idea.