It has the option of being used with a GPS with subscribers to Google Earth Plus, but I thought all the maps are streamed

Found some info on it http://www.kk.org/cooltools/archives/001623.php

Google Earth DOES work even when disconnected from the Internet. You just need to be sure the cache is updated in the area you are going to (you can set your cache to 2 gigabytes in the preferences to maximize this). So before I go somewhere I just zoom in pretty close and "fly" over the area or route, and this loads all the hi-res satellite imagery into the cache for off-line viewing. This solution is WAY easier than buying map CD's and downloading them into a GPS, and satellite imagery is way more useful for navigation.

I think the maps a streamed too... I'll hook up a GPS unit to my laptop tonight and test it a little bit, but I don't think it's just plug and play with Google Earth. Could be wrong though... I'll find out.