I have a USB device that requires more power than the RPi can provide. I don't want to use a powered hub. Couldn't I build a simple adapter that passes only the data lines and breaks out Vcc and ground to the device (no connection to RPi) such that the device could be powered by a separate source? Could the source be the same as the RPi's? Thanks for your thoughtful answers.
I would like to take this discussion one step further.
Apparently on the revison 2.0 boards, if you have a hub that puts power on a cable going into (coming from?) the USB A connectors you can power the Pi from the hub. In other words, the signal would be coming out of the A connectors and the power would be going into the A connectors. So in this case, the battery pack could be spliced into one of the USB cables. Or it could, for example, be incorporated into a keyboard assuming this application has a keyboard.
The resetable fuses protecting the USB outputs have been removed. This feature was implemented on some later revision 1.0 PCBs by replacing the fuses with links; revision 2.0 permanently implements this modification. It is now possible to reliably power the RPI from a USB hub that back feeds power, but it is important that the chosen hub cannot supply more than 2.5A under fault conditions.