Dapper repositories GPG error

Pete Shinners wrote:
> 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
> 

I had the same problem until I switched from the us. mirror to plain a.u.c

Jani