I guess this is an awkward suggestion, but thinking outside the box, it could work. I've seem some applications that use the movement of the iPod to track speed, such as an Accelerometer app you use to tell how fast you're going in a car. Through using Google maps to pre-load your entire trip, directions and all, would it be possible to make a GPS application that can be used completely offline? The accelerometer would allow the "GPS" to know how fast you're going, therefor keeping your directions/visual up to date. If the accelerometer messes up at one point or another, you'd be able to reset your position on the road/highway through the cache. Good idea? Bad?