Tappy Fish is unplayable. Hold down to flap, then the bird just shoots up to the top of the screen and dies. Don't hold and bird immediately plunges to the ground and dies...
I might check out Nightmode - that looks nice and seems like a simple enough tweak that it probably shouldn't cause any trouble. My iPad 2 is jailbroken, but I haven't yet jailbroken my iPhone 4. Going to do that soonish.