Mate, just like our buddy said right before me, you can't.
Part of the idea (if not totally) of Spotify is to focus music streaming in a way that will be more convenient than piracy, and at the same time to put it all in one single place, "to Spotify" it. Because music piracy works basically by copying audio files all around different devices and sharing illegally with other people, this is exactly the kind of thing you won't be able to do with spotify.
If you ever took a look at the local media files, you saw that those are not usual audio files, and I guess they aren't audio files only, at all. Probably that is encrypted data that will only be usuable if opened with a spotify client logged with a specific account synchronized to that file. That's anti-piracy methodology right there, one of the main reasons why I'm on premium. As someone who has been a musician for a time, I know that it means a lot to artists the fact that people are listening to the songs in a legal way, even though their profit out of spotify is not really significant. As for me, I fell relieved of not being the kind that download all the artist's albums through torrents just making the music industry even weaker at that point. (I know, I know, a lot of times they are just corporate bastards, but that's another debate)
Unfortunately, you won't get your spotify tracks into your iPod, unless Apple say that there will be a Spotify extension to it or hell get frozen; whatever that happens first will count. 😉