apt 0.6.43 not authenticating packages

[Les Gray <lgray@bigpond.net.au>]

Hi,

I'm doing a fresh install on my second machine and have just upgraded
apt to the backported 0.6.43. I then installed gnupg and added in the
apt-keys for debian-marillat and backports.org (debian archive keys for
2004, 2005, 2006 and AMD64 - don't why, I don't have one of these
machines - were of course already in there).

The trouble is, I'm getting a warning about non-authenticated packages
every time I try to install something, no matter which repository the
package comes from.

I'm only having this trouble on one machine. My main machine, on which
apt was upgraded a few days ago, is fine (although I notice the 2006
key for the debian archive was not automatically installed with it -
must be very new, I guess).

Can anyone help?

Thanks.