I would add the caveat that there should be a system added so that a person needs to log in each time they use it so that if the phone is taken by the police, they can not simply just gain acces to the phones messages (thus rendering it more a tool of incrimination). I'm not entirely sure how such a system would work, but I think that would be key to having it ported to android so that it didn't become a dangerous piece of evidence.
Shouldn't that also apply to your PC at home? ie. data file encrypted and unencrypted by password, just like the bitcoin client