Apparently, quite a bit.
http://chronicle.com/daily/2006/04/2006040704n.htm
[quote]Raymond G. De Vries, an associate professor of medical education at the University of Michigan at Ann Arbor, and three colleagues last year reported surveying more than 3,000 scientists about whether they had ever engaged in misbehavior, such as changing a study because of pressure from a source of funds, or failing to present data that contradict one’s own research. One-third of the scientists acknowledged they had committed some form of research misbehavior.
“Why are people engaging in these behaviors that they feel uncomfortable about?â€