System Information
Linux 64, nvidia.
Blender Version
2.70.4
Short description of error
Alpha is always = 1.0.
About 2 weeks ago all was ok.
Screenshot: http://i.imgur.com/50PKjDx.png
Blende file: https://dl.dropboxusercontent.com/u/26887202/blender/test_alpha.blend
Exact steps for others to reproduce the error
Just render my blend file. Alpha background must be = 0.0.