I tried browsing using the built-in browser (nasty app) to a particularly difficult page. Trying to exit the app at that point made my N-Gage hang up. It just displays the menu with the "Exit" option highlighted and won't respond to any keys, including the power off button. (Although it still responds to a USB connection, so possibly just the UI is shut.)
Is there a way of making it go again short of removing the battery? I'm hesitant about removing it while the phone is on.