apt-get update gave me this error on a squeeze box with virtualmin already installed:
W: Conflicting distribution: http://software.virtualmin.com virtualmin-squeeze Release (expected virtualmin-squeeze but got virtualmin-wheezy)
I started investigating and noticed that http://software.virtualmin.com/gpl/debian/dists/virtualmin-squeeze/Release has the following line:
Codename: virtualmin-wheezy
I hope that this is just a mistake and you did not abandon Squeeze yet?