Medibuntu is an 'accessory' repository for additional packages that are not part of the regular Ubuntu install. For the most part, it gives you additional media packages (MEDIbuntu) that are not open source. The fact that the GPG key is missing is not a big issue. It just means that you failed to list the GPG key when you added medibuntu to your list of repositories. The GPG key is a security feature that ensures that you are indeed receiving bona fide packages from Medibuntu.
The other issues I am not sure of. It may be that medibuntu is being updated, and the packages you want are temporarily not available. It may also mean that Medibuntu is moving on, and the packages for your version of Ubuntu are no longer available.
I've come across similar errors myself, using Kubuntu 8.04, which is getting a bit old. You don't mention what version of Ubuntu that you are using, but I assume that it is 8.04 from the error information listing it as a 'hardy' update.
Perhaps someone more knowledgeable than I can answer the rest of your questions.
Last edited by Frank616; 02-10-2010 at 10:14 PM.