SurferTim:
My bad. I understood that problem to include a voltage less than 6.6v applied to Vin also. If that is not the case, you have my apology.Well supplying Vin at less then 6.6v while also plugged into USB is operating outside of the boards stated specifications and requirements for the board, and indeed could cause problems. But your 'warning, danger, danger' did not couch it to this specific case but rather stated a global " has problems when powered by the Vin (barrel jack) and the usb simultaneously." which is not the case when supplied with proper Vin voltage clearly stated.
edit: Since you put your response to me so eloquently, I thought you might have forgotten to mention to the OP why IC7 would get hot.
I don't have a clue. IC7 is an op-amp and should not ever run hot unless it's defective or something on it's output pins is shorted to ground or Vcc. As the normal board current runs at less then 100ma, something inside or wired to IC7 is drawing 200+ ma of current that it should not be. Maybe the mosfet switch has a shorted gate and is applying +5vdc back to the op-amp's output pin. It could be troubleshoot with the proper methods and test equipment, but via a internet forum not so easy.
Lefty