apt-get update failing with signature errors

Hi all,

When I do apt-get update on debian4 I get the following.

Get:1 http://security.debian.org etch/updates Release.gpg
Get:2 http://ftp.us.debian.org etch Release.gpg
Hit http://security.debian.org etch/updates Release
Err http://security.debian.org etch/updates Release
Hit http://ftp.us.debian.org etch Release
Err http://ftp.us.debian.org etch Release
Get:3 http://security.debian.org etch/updates Release
Get:4 http://ftp.us.debian.org etch Release
Ign http://security.debian.org etch/updates Release
Ign http://security.debian.org etch/updates/main Packages/DiffIndex
Ign http://ftp.us.debian.org etch Release
Ign http://security.debian.org etch/updates/main Sources/DiffIndex
Ign http://ftp.us.debian.org etch/main Packages/DiffIndex
Hit http://security.debian.org etch/updates/main Packages
Ign http://ftp.us.debian.org etch/main Sources/DiffIndex
Hit http://security.debian.org etch/updates/main Sources
Hit http://ftp.us.debian.org etch/main Packages
Hit http://ftp.us.debian.org etch/main Sources
Fetched 107kB in 1s (96.0kB/s)
Reading package lists... Done
W: GPG error: http://security.debian.org etch/updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9AA38DCD55BE302B
W: GPG error: http://ftp.us.debian.org etch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9AA38DCD55BE302B
W: You may want to run apt-get update to correct these problems

Any idea how I can resolve this? I haven't changed any configuration - it just started happening out of the blue.

Many thanks.