Also, my sister called while this was happening. I picked up on her, the music stopped, and the Nav continued to give me commands while I was speaking to her. After she hung up, it went back to the Nav/Map and played my music again. Technology rules!
Just a note, the NAV only caches a few miles of maps, and doesn't cache alternete routes, so if you are on a long call, eventually it will give up until it has a data connection again.
Until we get LTE handsets (maybe early next year) we can't do simultaneous voice and data without WiFi.
Alan