key error message from squeeze-updates repository

[Lisi Reisz <lisi.reisz@gmail.com>]

I am getting the following error message when I run aptitude update:

W: GPG error: http://mirror.ox.ac.uk squeeze-updates Release: The 
following
signatures couldn't be verified because the public key is not 
available:
NO_PUBKEY 8B48AD6246925553

The debian-archive-keyring package is installed.

What has gone wrong and how do I correct it?  I have googled but found 
out only that the debian-archive-keyring package ought to be 
installed - and it is!

I am trying to get Squeeze fully upgraded before upgrading to Wheezy.

Lisi