Found myself installing Ubuntu 12 for a little project, and out of the box, post installation, apt didn't work, with apt-get update failing with error messages regarding "sources mismatch".
Sadly, sudo apt-get clean && sudo apt-get update didn't get things any better.
A little search brought me to an askubuntu discussion and solution (which in itself, was based on another discussion and solution by askubuntu member lorem). The solution is remarkably simple - delete all of apt lists, and start over:
sudo rm -rf /var/lib/apt/lists/*
sudo apt-get update