Just wanted to share a problem that I came across but have fixed incase anyone else has similar problem.
Basically I had an extra texture render element with nothing plugged into the slot. This made my beauty render pass and several others come out totally black but with an alpha.
It had me baffled and only figured it out with a process of elimination, after half a day!
If you keep the scene around, it would be useful if you can send it to me at vlado@chaosgroup.com so that we can fix it. Other than that, I could not reproduce this in a simple test.