I was just wondering if anyone with iOS 6 installed has had a chance to try the above out whilst on a trip. Thanks.
I've done it. I didn't notice the screen immediately after the call came in but either I changed it back to the maps or it did so on its own (I'm guessing the former).
I was using a headset and Siri kept piping up with the next directions while the conversation was going on. The caller on the other end could not hear her (or didn't mention anything). Since I find that Siri gives a lot of updates it was a little annoying but tolerable.
You can control the volume of voice control in Settings->Maps