Knowledge@Wharton has an article confirming what we, at SelectMinds, knew, but had a hard time quantifying with empirical data.

“Contrary to the view that companies lose something when a worker
leaves, the study found that they stood to gain. Specifically, firms
that lost an employee to another firm were 8% more likely to cite that
firm than other equivalent firms, Rosenkopf says. The reverse flow of
knowledge was particularly pronounced when the employee moved to
another region. Then the old firm was 22% more likely to cite the new
firm.”

Via David Teten.