Moxie Marlinspike does it again. The eclectic hacker who previously brought you SSLStrip now has released (@ Defcon 20) a utility and advisory on cracking MSCHAPv2 which powers most PPTP VPN.
Get the code here: https://github.com/moxie0/chapcrack
Suggestion is to migrate to OpenVPN for a more secure VPN setup.
Also if your bored read some of his excellent stories