Comment 10 for bug 372923

found that was caused because I needed python 2.5 and installed using
deb http://ftp.debian.org/debian/ lenny main contrib non-free
in my /etc/apt/sources.list file from a recommendation online

OUCH
remove that line do a
sudo apt-get update

now all is well again

If all possible it would be nice for update-manager to be able to recognize these problems easier and inform the user