MightyMonitor Widget displays the power status of your mouse and keyboard in a simple Dashboard widget that changes color according to how low the batteries are and indicates how much power is remaining in a couple of dials. There's not much more to it than that. The only option is whether to turn off either the keyboard or mouse monitor if you only have one wireless device. It's a shame it doesn't support other wireless devices that require batteries but for Mighty Mouse users, it's ideal.
MightyMonitor Widget is the solution you need if you always find your Mighty Mouse and wireless keyboard are running out of battery without you realizing.