"Bad GPG Signature" error for Ubuntu repositories

[zer0halo]

Just yesterday I started getting a "bad GPG signature error" when
"updating" (apt-get update) from Ubuntu repositories. For example:

W: GPG error: http://archive.ubuntu.com hoary-updates Release: The
following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu
Archive Automatic Signing Key <ftpmaster at ubuntu.com>

However, 
$ gpg --list-keys ubuntu
results in:

pub  1024D/437D05B5 2004-09-12 Ubuntu Archive Automatic Signing Key
<ftpmaster at ubuntu.com>
sub  2048g/79164387 2004-09-12

So I do have the key in my keyring. When checking the key properties
(using Seahorse), it shows the signature key ID as being
40976EAF437D05B5, so it does match). Therefore I don't understand why
apt-get is giving me an error unless there's a problem with the
repository? Any clues? Thanks.

-- 
"Impossible is nothing."