There's a top tier DEFCON talk by the Lavabit email guy. He explains where the line is for access to phones and other encrypted information. I'll try to summarize -
1 - Law enforcement have actual information about the probable contents of your phone (like an incriminating filename will do). They can reasonably expect to get a warrant and access to your stuff.
2 - They don't know what's there at all, and have no probable indication of the contents, and in this case they cannot expect access because they would just be going fishing.
