Your phone should automatically stay awake while using a navigation program. Mine does using Google Maps/Navigation. Same with Sprint Navigation. I just checked and I don't think there are even options in either program to NOT stay awake.
Both Sprint Nav and Google Nav keep the screen on for me.
However, if you want a widget that will keep your screen on, check out SwitchPro in the market. That is one of the many switches you can set in that excellent widget.
Here's a screenshot with a terrible choice for a background, but you can see the various looks of the widget.
I use an app named quick profiles. While navigation does keep my phone on I also want it to be always on when playing music in the car or in the car in general so I dont need to press the button and unlock it. I created a profile for 100% brightness and no screen time out and then one with my normal settings. Works great. Best of all the app is free.