Google Earth exists for precisely this purpose: overlaying your own data on a map. You simply write scripts to translate (or export) your data into KML format then add the URL to Google Earth like so:

(source: woopra.com)
With regard to terms of use: the terms of use for Google Earth only forbid you to use the software to control the behavior of your vehicle, not to represent its past behavior. (Their concern is most likely to limit their liability from you trying to draw a path on a road in GE for a robot to follow, and having it crash into something due to a map inaccuracy.)
As quasi-proof, I wrote a mission planner with Google Earth at one point (http://www.youtube.com/watch?v=vK0wWH9Ijnk) which I showed to the Google Earth team; they were fine with it. They also put me in touch with a team at NASA that uses Goole Earth for this purpose. I'm led to believe that such usage is allowed since I'm only generating mission scripts and not interacting with the autonomy.