Seems doubtful that Intel Macs would handle this stuff differently than G4-based Macs, but I suppose it's possible.
What's odd to me is that the connection terminates where it does - once you've successfully authenticated, the Mac PPP stack takes over and the script exits. So for you to get an LCP termination message, you have to be able to successfuly send data over the link!
Since you're already looking at the log, here's what you can do to get more debug output (please be comfortable with UNIX terminal commands before even thinking about attempting this, and don't do it if you've set up other ppp options!): in Terminal, execute the following:
sudo echo 'debug' > /etc/ppp/options
You'll be asked to type the password you use to log into your Mac. Afterward, try to connect again. You should see extra logging between PAP auth and LCP termination. Again, don't try this unless you're sure you know what you're doing, as mucking about with sudo can seriously hose your mac.
One further note - those whose connections are terminating after less than 10-15 seconds I can probably help, but anything longer than that is most likely a signal strength or interference issue. Still, I'll help if I can.