Comment 67 for bug 24234

I just ran into the same problem.
Switching form the country server to the central server seem to have worked around the error message.

#Example of change in sources.list after Synaptic repository changed to "Main server"
http://za.archive.ubuntu.com/ubuntu/ ->> http://archive.ubuntu.com/ubuntu/

#Error before:
The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <email address hidden>

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 9.10
Release: 9.10
Codename: karmic

Not a ideal situation.