iToaster, I don't mean to rain on your parade, but if I were you I'd get rid of that thing, and fast. I really can't see from the picture what kind of mouse that thing is (looks like a run of the mill field mouse), but those little bastards are vectors for all sorts of diseases. Out here in the West we have deer mice, whose droppings can contain a strain of the Hunta virus (nice disease, turns your brain to liquid crap... and then you die). Mice in the lower elevations will almost always carry fleas, which will be a pain in the @$$ to get rid of, and they can infect your cat too.
Yes- it''s good that you didn't let one animal kill another, but instead of taking pictures, I would have chucked him far away from my house.
That's just me- I'm a pessimist.