Dunno bout the Touch Pro as I haven't done this yet, but when I had my Touch and I had it plugged in while the GPS was running with Live Search the phone got really hot and stopped charging. I would unplug it and put it by the AC and it would cool off and charge again. For the rest of the trip I could get it to charge by leaving it by an AC vent so that it could keep cool.
Is your phone getting hot when you're doing this? If not then... well I told you a nice story
