Let the hive mind of Engadget get that for you.
"As someone who doesn't reside in the USA, I was wondering what would be the best way to get internet for my computer in the US for a couple of weeks? If it were Europe I know I'd look for some prepaid data. Is there anything similar offered by American carriers? A MiFi or a data SIM that I can tether from would work, but I'm trying to maintain a tight budget. Help!"
Regarding the update: you are getting warmer.
The data isn't exportable from the in-vehicle navigation system. You download it from the Internavi website and then view it in Google Earth. Go ahead and try it. Go to this page and click on one of the three buttons labeled Tokyo, Nagoya or Osaka and download the "kml" file.
http://premium-club.jp/PR/lab/lab1.html
If Google Earth is installed on your system, just double click the file after it finishes downloading. The data is updated three times daily.
Kind of anti-climactic, isn't it?