The Ubuntu release upgrader should deactivate any apt-pinning definded in /etc/apt/preferences or /etc/apt/preferences.d/* and notify the user when doing so. A similar behaviour is already the case when external (PPA) sources are deactivated.
Otherwise, some random irritating error will be shown which does not show the origin of the problem. In my case: "kubuntu-netbook cannot be installed".
I use apt-pinning a lot and I know what I am doing. But when upgrading, I usually forget that I have pinned packages or sources and it takes me some time to find out what's wrong. It would just be nice to warn the user and/or deactivate the pinning in /etc/apt/preferences/*
ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: ubuntu-release-upgrader-core (not installed)
ProcVersionSignature: Ubuntu 3.2.0-32.51~ppa1-generic-tuxonice 3.2.30
Uname: Linux 3.2.0-32-generic-tuxonice x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu14
Architecture: amd64
Date: Fri Oct 19 13:38:00 2012
InstallationMedia: Ubuntu 10.04 LTS "Lucid Lynx" - Release amd64 (20100429)
ProcEnviron:
LANGUAGE=de_AT:de:en_US:en
TERM=xterm
PATH=(custom, user)
LANG=de_AT.utf8
SHELL=/bin/zsh
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to precise on 2012-10-19 (0 days ago)
Thanks for reporting this issue and helping in making Ubuntu better and sorry since it has been a while since you reported it. I will post your request to the bug-team to see if your request can be considered as a valid feature request and if work can be done for it.