Except that text history is usually saved on the phone. If someone has access to sending texts, they will have access to that as well. It is probably better to go with phone locking, instead.

That said, I like the passcode idea. I know that caller ID is easily spoofed, and so I would imagine that the source ID for text messages can be spoofed as well. Requiring the passcode would prevent this sort of attack.

that may not be ideal. most people keep their sms history, so the person trying to use the service in my behalf could just scroll the screen. or do you mean some sort of one timer passcode? that could complicate things a bit, not sure how it could work