The devs themselves said they are aiming for 60 fps and 3D ! They themselves are responsible for this "rumour" ! They were even open to Move at that point !
Then at the very least i'd expect an absolute rock solid 30fps if they were even serious about hitting 60fps. Who knows, maybe at some point they DID hit 60, but without any bells and whistles on. I imagine it would of looked a bit empty and sterile hence the shift to 30 with carnage on screen.