Navigation is one of the iPhone app categories that will really benefit from multitasking, and Navigon will shortly release a free update to provide that highly desired functionality.
You can leave the app when a call comes in, and when you end the call you'll still be navigating. Better yet, any voice alerts will be heard while you are on a call.
The US and Canada version will be updated first, with the regional versions of the apps to follow shortly. Navigon has continued to be very aggressive in delivering features, and this latest update will be an important leap forward. No release date has been announced yet for the 1.6 update, but it's in review with Apple now.
If you'd like to see the new features in action, Navigon has posted a video of the new version in operation. I can't wait for this update, and I expect I'm not alone. TomTom has also announced it will be supporting multitasking shortly after the new OS is released.