I just spend like half an hour to find out why I got a black render…
And it was because I had pressed (by accident) the do sequence button.
And I wasn’t even rendering an animation.
I had a black render in yafray and the internal.
No idea if this is a bug or just me, but i think it is good to know (saves you half hour on finding it) 
Hello… got even stranger!
See here: http://blenderartists.org/forum/showthread.php?p=1121265#post1121265
No more black… but a weird strange image… DingleDong! DingleDong! I’m driv’n crazy!!! :o
the do sequance button enables the sequince editor (press ctrl+left/right a few times). if there are no strips then the output will be blank. Its a feature, not a bug 