It's probably in your apt sources. Open Synaptic and take a look under Settings -> Repositories
to see if there's anything related to Spotify. It looks like there should be at least one entry, and it's possibly outdated or shut down and can't be downloaded via the entry in your repositories (that's what probably gives the error about not updating some index files).
Also, part of your error has to do with GPG signatures. Probably at least part of that is related to Spotify, and it may go away completely after you solve that part. But there may be others. I've had those before and never worried about them because I always added the repositories manually and knew they were safe. But if that isn't how you added them and you're worried about trusting them, you'll end up having to research the repositories and get the proper commandline stuff to download and store the signatures. It's usually not too hard to find if it's not something obscure. And adding them is easy peasy.EDIT:
I didn't see you'd already fixed it before I answered. Next time I'll make sure I got to the end first.
Edited by stiltskin, 28 February 2012 - 10:08 PM.