I recently tried out a friend's battery charger case and my iPhone 3GS began to buzz and flash between locked, wallpaper, and temperature warning screens.
Not to be all Wall Street Journal about it, but according to sources in a position to know, this is not an uncommon problem and Apple is set to address it in the next firmware update, which should be coming soon -- 3.1.
3.1 you ask? It's what we asked as well, and were told the iPhone 3GS is already running what's considered 3.0.1 and the next update would likely be 3.1 (though 3.0.2 or an "official" cross device 3.0.1 still sounds more like as a bug fix release to us).
Consider it a rumor for now, but at the very least, if your charging case is giving you headaches, rest assured help is on the way.
(And it's recommended you don't use it until the problem is addressed).