Some games might make the transition relatively easily. A game that relies mostly on single points of touch has a somewhat decent analog with the mouse, but only if it relies on certain patterns of finger movement. A game that, for instance, involves alternating taps on opposite sides of the screen, would translate badly—it's nowhere near as easy to leap that distance precisely with a mouse as it is with your fingers. Can you imagine playing the iPhone classic Dactyl with a mouse?

That kind of difference is present all over in touch-based applications, in differing amounts. It's up to the developer to find what transitions well and what doesn't, but in most cases it won't be a trivial task.