I'm confused about the different versions of the CMMI. Which one should we be using right now?
I understand your confusion. In version 1.1, there were multiple "disciplines" within the v1.1 framework. Software, Systems Engineering, and Supplier Sourcing were all available. When v1.2 was released in August of 2006, you could choose v1.1 and an accompanying discipline OR you could choose CMMI - DEV v1.2. As of August 2007, CMMI-DEV v1.2 is the only version of the CMMI that is active and supported. The concept of disciplines has been eliminated, and the "DEV" refers to the only "constellation" that is available ("Development"). Other "constellations" are planned for the future (Acquisition, Services) but have not been released.
So the answer is simple - there is only one choice right now, CMMI v1.2-DEV.