I'll start with a stupid question: If USB provides +5V power, why does the DC power input jack need 9-12 V? Won't the 78M05 voltage regulator burn most of that up?
It's a good question... the basic principle is that most regulators need a higher voltage than they put out, as they have cicuitry that needs to be biased in order to operate. In the Arduino, the DC jack needs to be at the minimum input voltage for the regulator, plus the voltage dropped across the blocking diode. The 78M05 needs about 7V (the internal circuitry has to have some voltage across it to operate), and the diode drops .7V. So you need 7.7V <<minimum>>, or about 8V+ to be stable. When you operate from the USB power supply, you use the computer's regulated power supply.
The 78M05 regulator does have efficiency losses, but they're small... It's quiescent current is under 10ma. When you draw current from the 78M05, you only "burn up" as much as you use: e.g. converting from 8V to 5V at 100ma you'll dissipate 300mw of heat in the regulator (more or less, approximately, roughly, so to speak, etc). Small price to pay for it's cost and simplicity.
Re aaronprobst's post about wanting a 12 supply: just get a gel cell battery (the kind used in emergency lights.) 12V, rechargeable, fairly cheap.
This gel cell
, for example, will supply 100ma to a load for around 60-70 hours. Use the 12V for high-current loads like motors etc, and to power the Arduino through the DC in jack...
Hope this helps,