Dapper repositories GPG error

I've been unable to update my Dapper repositories recently.
I'm sure there's a way to update my keys, but I'm not finding anything
through searching or man pages.

$ sudo apt-get update
...
W: GPG error: http://us.archive.ubuntu.com dapper Release: The following
signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic
Signing Key <ftpmaster at ubuntu.com>
W: You may want to run apt-get update to correct these problems