Whatever power supply I would use it should end in a micro usb connector. I first stripped a cheap cable and then soldered 2 wires. You can find the pinout here. Don't worry about the data pins since we'll only power the rPi.
Perfect for a raspberry Pi? Not at all! They only last for about 30 minutes. The voltage then dropped too much and the USB dongles stopped working. The power led on the pi still blinked but no USB == no wifi (wireless remember?).
I then explored lipo power. I have several batteries laying around. Just needed a 5V regulator. Not those 3 pin thingies that get hot like hell wasting all the power as heat.
The fun solution is to find a cheaper DIY alternative. I found something referenced as MP2307 (just search it on ebay, it's all over the place). According to the datasheet we can use this to regulate input from 4.75V up to 23V to a more rPi friendly 5V (anything from 0.925V to 20V). The below image shows how to use it.
The components look very similar to those commercially available UBEC. And guess what, you can ignore the modifications listed in the pic on top and just solder your connectors and a 10k pot. As soon as you power it up and change the voltage to something around 5V the black part in the middle that you should have removed will pop.
Now I just need to finish with some shrinkwrap and I have my lipo powered Raspberry Pi up and running. BTW don't forget that lipo batteries don't like being over discharged (or over charged, or dropped, or being abused in any other way).