Developer Mach Kobayashi has stepped up to fill this need with his $0.99 dice simulator called Mach Dice [AppStore]. Mach Dice also provides a realistic dice simulator with some distinct advantages. Specifically, you can choose how many dice you want to roll and how many sides (up to 100) each die should have. Kobayashi's app also seems to have a higher accelerometer sensitivity and a more appropriate response to directional shaking than MotionX.
The video shows it best: