What is the significance of this and should I be concerned?
<W: GPG error: http://archive
.com lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5>
By the way I'm running LMDE
I'm guessing you added an Ubuntu repository or PPA for some app available of LM 9 - Isadora or Ubuntu 10.04 - Lucid Lynx because of the address of the archive. LMDE will not be prefectly compatible with Ubuntu since it is based on Debian instead of Ubuntu. Is that correct? I guess you could have edited your sources.list in some other manner.
From the LMDE release announcement
FAQ about LMDE
1. Is LMDE compatible with Ubuntu-based Linux Mint editions?
No, it is not. LMDE is compatible with Debian, which isn't compatible with Ubuntu.
2. Is LMDE fully compatible with Debian?
Yes, 100%. LMDE is compatible with repositories designed for Debian Testing ...
If you happened to have added a PPA which will work with LMDE then look to the PPAs instruction page for the Signing Key. You should be able to adapt these instructions
for adding the key.
Step 5: Back on the PPA's overview page, look for the Signing key heading. You'll see something like:
1024R/72D340A3 (What is this?)
Copy the portion after the slash but not including the help link; e.g. just 72D340A3.
Step 6: Now you need to add that key to your system so Ubuntu can verify the packages from the PPA. In your terminal, enter:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 72D340A3
Replace 72D340A3 with whatever you copied in the step 5.
This will now pull down the PPA's key and add it to your system.