Thanks for the info.
I had half expected to have to manually obtain a new key, however after running apt-get update/upgrade again today the message no longer presents.
Upon checking the keys:
pub 1024D/4E7532C8 2011-01-11 [expired: 2012-01-11]
uid Opera Software Archive Automatic Signing Key 2011 <firstname.lastname@example.org>
pub 1024D/336064B5 2011-11-08 [expires: 2013-01-11]
uid Opera Software Archive Automatic Signing Key 2012 <email@example.com>
sub 4096g/120EDEE4 2011-11-08 expires: 2013-01-13
It seems that a new key has been provided.