Some time late in the development cycle, we need to unteether the power cord and set the ARduino free and portable. I found in this regard the Arduino design has fallen short. The "preferred" method is to plug in a 9V battery thru the DC power jack. While this approach is acceptable, it is wasteful: it drops the volatge from 9V to 5V, a differential of 4V. If the board and pluggged in shields draw 200mA. That's a whopping 800mW burnt up in heat! Also the FDTI chip is drawing needlessly current since the USB subcircuit is not needed. I wish the future reference design will incorporate a double pin header for external 5V, and a switch to disable the FDTI chip in portable mode.
In the mean time, I found a good way to incorporate a battery pack to power the Arduino board:
bodhilabs.com/info.html
For all the reasons you cite, they created Arduino Pro, see here: