I've wondered, but there are two reasons why I'm not a total believer in the "he's faking amnesia" theory: First, he consummated his relationship with Edie, and they always paint Mike as the morally superior criminal. (Classic bad-guy-with-a-heart-of-gold stuff.)
Second, he seemed genuinely interested in finding out "what he was like" from Carlos and Tom.
So, I think, for now, it's real, but I suspect that at some point, he will regain his memory and continue to pretend that he hasn't. It's then that he will be in torturous agony over the death of Monique (wasn't that her name) and Susan. He does "tortured agony" well, so given the chance to write that in, I think we can safely assume it will come to pass before the season is over.