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.

Having said that - backdoors are bad.