using a local mirror and an official mirror of -backports does not work

In the event that you use a local mirror but do not mirror -backports you will end up in a situation where you are not able to perform a distribution upgrade.

An example sources.list:

deb http://192.168.1.1/ubuntu feisty main restricted
deb http://192.168.1.1/ubuntu feisty-updates main restricted
deb http://security.ubuntu.com/ubuntu/ feisty-security main restricted
deb http://archive.ubuntu.com/ubuntu feisty-backports main restricted universe multiverse

What will happen is that the first two lines will be commented out by ubuntu-release-upgrader and it will check -backports for ubuntu-minimal and will not find it and the upgrade will exit. You will be presented with an error message that says:

'This may be because you have no official mirrors listed in your software sources, or because of excessive load on the mirror you are using.'

However, if things were working properly you should instead see:

"No valid mirror found … Do you want to rewrite your 'sources.list' file anyway?"