Ubuntu 14.10 - GeForce 840M
Broken: 2.73a, master
Short description of error
No alpha blend in cycles viewport, should expose BGE alpha blend setting in cycles UI, since it affects cycles too, and its the only way to set the blend mode in cycles viewport.
Exact steps for others to reproduce the error
Just open the file, the plane texture doesn't show alpha
Go to Blender Game, and change the alpha blend setting from opaque to alpha blend.
If you go back to Cycles, the texture is displayed correctly.