People are responsible for their actions
Anyone handling sensitive data needs to be comptetent enough to look after the data in their care. This isn't about Limewire or other P2P apps but encompasses much wider considerations like the use of email (including private accounts e.g. Palin), USB sticks, notebooks, PDAs, mobile phones, cameras, etc. People who can't grasp the importance of the issues involved probably should not have access to sensitive data, at least, not in a form which they can copy or otherwise disseminate.
What I don't understand is how IT policies and auditing in the organisations concerned allow users to install and run applications like Limewire on systems which can access sensitive data. Are US government systems containing classified documents really allowed to connect with the Internet and/or do they allow files to be copied to other systems or media?