From that same website it looks like the delorme gps uses the DTR line to detect whether they are connected, but that still doesn't explain how the streetfinder figures it out. http://www.gpspilot.com/Palm-GPS.htm

It would be noce to know, maybe it doesn't really know whether it is in use or not and just chooses the 'safe option' (i.e. stay on). My guess is that when it is idling it should be able to keep it's ram fresh for a really long time on the battery.

more interesting info, the device stays on when not running off of the battery. http://www.cetusgps.dk/info/faq.html#rand