So you're going to end up with something like this:
**broken link removed**
The relay is not going to be a problem. But if there's any place where the danger of getting 12 volts into your iPod is, it's at the jack. So it would pay to get a decent-quality jack, and definitely test it before plugging anything into it. Just put a plug in and put a voltmeter on both contacts and make sure you read zero volts there.
If you're still worried about this, you could put two capacitors in after the jack signal lines to isolate it in case something goes wrong. 10 µF ought to do it (you can get non-polarized caps if you're concerned about them being reverse-voltaged).