While playing around with a few things I have on the go at the moment, I found that quit often I was only using the Arduino to provide power for the project while I mucked around with the components. Then a though struck me; I have a couple of really nice little 7.4v lithium polymer batteries that I use with my model helicopter and wouldn't it be great if I could use these to power my projects!
One trip to Maplin later I had acquired a TS2940CZ50 1A Ultra Low Dropout Fixed Positive Voltage Regulator. Yeah.
Basically this little gizmo looks like a transistor with a nice big metal tab on the top, and when used in conjunction with a couple of capacitors provides a regulated 5v DC output from an input voltage of up to 30v DC.
Next step is to read the datasheet (available from alldatasheet.com) and then using the sample circuit on page 3 I built this little thing on a scrap of vero board I had lying around:
Whilst reading the datasheet I noticed that the voltage regulator requires a minimum current draw of 10ma to ensure proper regulation (something to do with leakage currents dominating the output and driving it high) so I added an LED and a 560ohm resistor, providing a current draw of about 8ma, the idea being that adding anything else at all to the circuit will give a current draw of more than 10ma.
The other thing I have done is to use a standard two pin header and push the pins right through the board so that they stick out the other side (you can see the top of the header protruding form under the LED). This means that I can plug it into the breadboard like this.
So there we have it, a functioning 5v regulated PSU.
The only modification I want to make for the future is to include low voltage protection for the battery, as li-po's do not like going completely flat. In fact they like it so little that they die.