Is LUKS going to help here? If the phone is switched on then the LUKS keys are held in memory and the disk is completely open. All that an attacker needs to do is to ensure that the phone doesn't switch itself off or run out of battery while the information is copied off.
Indeed, I meant to address this in my original post. It is not fool-proof but in most cases it's reasonable to turn your phone off after getting pulled over or before meeting a security checkpoint. Certainly much, much more secure than what we have now.
