I actually just thought of using a battery like this:
I know it's a bit expensive, but it may be essential for at least one of the projects I'm doing with the RaPi! It's not really a "real" ups I know, but it may just work as one. I know the RaPi will not know wether the power is off or not, but for my use that will probably not be that important. I think it will run for 20-30 hous off that battery.
Just remember that 8800mAh is not probably what you get out from a device that weighs 106 grams
This appears to be a battery charger, not a UPS, no?
This is only the schematic for the charger side. Attach a wallwart, battery and a 5V regulator, and there's your UPS for Rpi.
I wuold guess that most people only need 5V and for that purpose devices like the one on eBay link are just fine. This schematic though can be used if you need e.g. 12V or 24V for something that needs to run with Rpi.