The signing key for the MEPIS package pools expired Jan 22, 2011. Who knew we would be around so long?
The attached file is the new public key that you need in order to install MEPIS packages.
To install the key:
Download the key file into a directory of your choice. You may need to right click on the attachment link and select something like "Save Link As..."
In the download directory, open a terminal as root and run this command: apt-key add MEPISdev.asc
This will install the key on the apt keyring.
Then when you use apt or synaptic, the MEPIS pools should work normally.