There is a thread about this at http://forums.adobe.com/thread/493752, which has been closed because it claims the question has been answered. It was not answered, and is now useless because the originator unfortunately used the wrong terminology in her question - using the term "merged" instead of "locked" - and just gave up when the answers were unusable instead of clarifying her issue. There is also nothing whatsoever in the help files addressing unlocking a locked layer.

Just so you'll know right from the start, this is NOT about background layers, so please do not respond here by telling me I must double-click and rename the background layer to unlock it.