I was wondering what would happen if you plugged in a wall-wart while the 9v battery was still
connected. It's not clear from the schematic how the plug and header are wired together.
On my freeduino board, the 9v vin pin is connected to the input of the voltage regulator. The dc in socket connects to the regulator input through a diode.
So, in your scenario, the wall wart voltage (less the .7 diode voltage drop) would be applied across the battery. This is not a good idea. But if you connected the battery via a diode (similar to how the DC in is connected) then whichever had the higher voltage would provide power to the regular.
how much current can be drawn off of the 5v header pin?
Depends on the regulator on your board. Could be anything from 250 ma to 1 amp. If the part numer of the regulator is legable then you should be able to find the specs on google.
I hope that helps