There is a program called Keyboard Locker that has that option when using a GPS application. It also has the option to lock the keyboard after a specific time (user defined). So you would disable the lock when holstered option and turn that option on. You can get the application at Welcome to BEIKS mobile!
I don't know if it really prevents the screen from fading while using a GPS application, you will have to read the info about it to see which applications it "respects".
Oh, did I mention that its FREE????