I have reproduced this on 5.2 and 6.0 and have not tried
6.1. run this:
The connection cannot be brought down.
We can't really fix this in the old versions; that's why we significantly
re-architected how we do PPP connections for 6.1. We now have a "ppp-watch"
daemon that handles all this stuff and makes it reliable. It has many other
benefits as well, such as making "ifup ppp0" synchronous.