Found this today, looks very interesting to me, should serve in a lot of scenarios. It even comes with clean power off/on switch, so actually its 2 solutions in one product. (needs drivers to be installed though, but they are supplied by the company)
This is what they say:
The Pi UPS + (Uninterrupted Power Supply) device developed by CW2. guarantees the operation of your single-board computer by using rechargeable batteries. In the event of power failure the Pi UPS + can either shut down your device properly or temporarily continue keeping it running until external power is available again. So there is no risk of data loss and system damage. Running applications on your mini computer are thus permanently protected. The only thing you should look after is the status of your batteries which is possible by using the bundled monitoring software.
If the Raspberry Pi is used in a car to connect electronic equipment via USB ports, a proper shutdown is guaranteed as soon as the engine is turned off. Whenever the ignition is restarted, the Raspberry Pi will reboot too.
If the Raspberry Pi is connected to a TV, e.g. a XBMC Media Center with Internet access, all currently running applications will be shut down and terminated properly as soon as the TV is turned off. Once the TV is turned on again, the Raspberry Pi restarts too.