Yes you can supply the RasPi through the USB port. There four lines in USB 2.0 port, 5v-,5v+,data- and data+. There is a limit in the specs at 500ma but there are non spec devices that use more than that. You also can use a "Y" connector it has one plug with four lines for full USB and one plug with only the power, this allows a draw up to 1amp off of two USB ports. the RasPi is reported at 1watt draw, a single port can supply 2.5 watts. If you power the RasPi through a hub you will not be drawing power through the RasPi. Of course in the B model you have two ports, if you use USB to power it you would not be able to draw power for a device from the second USB. Drawing current for the device might overdraw and leave none for the RasPi. This would be an unstable arrangement at best.
I am hoping for the 12v but can work with it either way. Anyone have a diagram or a link to a board that is a nice, safe 12v to 5v drop? Probably something in spark fun I would guess.
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(RS)Allied ships old stock to reward its Customers for long wait!