I have noticed that long clicking an SMS message brings up a menu containing a "Lock message" option. What does this do? I have a Nexus S.
If you lock a message, when you delete a conversation with a locked message, every message other than the locked message will be erased from that conversation.
On most android smartphones there is a feature that makes the phone run faster. This feature is after a certain amount of messages(on mine it's 210)it begins to delete them,locking the message also prevents it from being deleted.
that means (normally on every phone) that it cannot be deleted, just like a locked picture on a camera. so, if you would like to keep a message safe, i suggest locking it so no accidents happen. although, when the time comes that it wants to be deleted, it can be unlocked and then deleted