Actually, there are a couple of vrm tools for the N900 already setup. They're more designed to overlay images on pre-entered ID tags in 3d, but they could easily be augmented to do this.

Why you would want to do this is beyond me. Concentrating on a small device while trying to move is just asking for trouble. Some things can be multi-tasked depending on what the task is. Walking and driving are rarely in that list.