good idea. much more should be done in this area. however there are several very hard problems though. proof of balance is not worth very much if an attacker can remove all coins in minutes. so if 500M$ evaporate without noticing, there is the question how to create accounts which are more secure by default. could one create an address which has different features? an address which is sealed so that only a certain percentage can be withdrawn? a multi-signature address? everything depends on the security mechanism.