This is a very short HOWTO that answers something that's been bugging me for months. Network Manager was finally working fine with my Debian laptop's wireless, but would not (as advertised) manage its wired connection. The solution was found in an obscure email list conversation. Go to /etc/network/interfaces and comment out the lines related to eth0. Then restart Network Manager or just reboot.
I have no idea how you're supposed to know this. (B)