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.
It should be fine. You should connect the grounds together though.
If the wires are really long, you may have problems with earth loops
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.