Let me count the ways:

18.4% - Theft by employees
18% - Hackers
14% - Lost or stolen media
11.6% - Personal data inadvertently exposed

Kind of drives home the importance or proper access control, doesn't it?

Source:
http://voices.washingtonpost.com/securityfix/2009/06/malicious_attacks_blamed_for_m.html?wprss=securityfix