From http://research.microsoft.com/apps/pubs/default.aspx?id=1618...
Of course, as that paper points out, the traditional electronic money system is incredibly reversible. If someone transfers $50,000 from my personal bank account to someone else's bank account, it's pretty easy for it to be undone.
The bottleneck is the money mules who are hired (read: suckered) into engaging in irreversible transactions.