Headphone level pretty much is line level, maybe a bit hotter but able to source more current. A regular "iPod to home stereo" sort of cable should do the trick.
It works the other way, too. One of my system troubleshooting tools is an adapter that goes from female XLR to female 1/4" TRS, so I can plug my headphones right into an XLR output. It's not especially loud, but it works.