Didn't work for me. I removed the VPN entry in the Network system pref panel, then removed the keychain entry associated with it. Then added a new PPTP VPN configuration, and entered the password by clicking the 'authentication settings' button. This caused a new 'PPP Password' entry in the keychain.
However, after doing all this, I still get an 'Internet Connect' titled window prompting me for the username and password - along with an icon for the Internet Connect.app which no longer appears to exist under Leopard. If I enter the same password that is already stored in the keychain, I get a connection. If I don't, then it fails with ;
pppd 9233: MS-CHAP authentication failed: E=691 Authentication failure
..in the syslog.
Me thinks a bug affecting those who did an upgrade from Tiger.