I reported in the Navigon 1.2.0 update that there was a new feature called iPod controls which allows you to fully use the iPod controls from within Navigon.
I have to give credit where credit is due, Navigon have implemented this pretty well. Once turned on in the OPTIONS you just tap the ORANGE iPod at the bottom right of the navigation screen, and then the navigation screen starts to peel back as in image 2 below, and then fully shows the iPod control screen. The peeling back of the navigation screen is a nice feature, but a little gimmicky.
Once you’ve done what you need to on the iPod control screen you can tap the MAP button at the top left and go back to navigation. I think what Navigon have tried to prove is the iPod app is always running in the background in Navigon by peeling away the layers.