I've modified my sources.list file manually and when I ran "aptitude update" I was told
W: GPG error:
http://www.debian-multimedia.org lenny Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907
W: You may want to run apt-get update to correct these problems
So I ran "apt-get update" and got exactly the same reply!
All this was actually working last week, and I installed three programs. What's happened since then? More importantly, how do I get out of this mess?
[RANT] I can remember installing Fedora Core 1 — the first time I'd installed an OS. I had everything working in a couple of days. Now, with several years extra experience, I've been trying to get Debian sorted for a couple of weeks! And I thought its reputation as an unfriendly distro for professionals or hobbyists must be exaggerated… [/RANT]