In general the source code is just a bunch of text files. Whoever has access to them, can ,,steal'' or ,,pirate'' them, regardless of whether they are stored in CVS, git, Subversion, Mercurial, Windows shared folder, etc.
Distributed version control systems make it easy to grab a copy of all the history, but a hypothetical disgruntled employee can grab complete history from CVS server too. Or from backup tapes, or by manually copying the files over the years.
The only thing you can do is to limit copying all the data in your organization.
- iPods
- removable USB hard drives
- upload to untrusted web sites on the public Internet
- SCP/FTP access to servers outside your company
I wouldn't worry all that much, unless your work really needs to be super-secret (think military and such). If you treat your employees right, they will have no reason to hurt your organization.