I have two planes with the same node setup, but only one of them is transparent in Material Viewport Shading.
The one that works was created by the Import Images as Planes addon. The other plane was created manually and copying the addon's material setup into a new material.
If I set the addon's material to the "bad" plane it works, so the problem is related to the material itself (nor the object, OPGL or my hardware). I'm working on 2.73.
I have no clue about this behavior, here is the .blend for checking it out.