Hello all,
I would like to know what would happen if the dc supply of the arduino is supplied by -9v instead of +9v. I am building my own power jack and i am unsure of the jack's polarities.
Thanks. 
Provided that you have a genuine Arduino, with a diode in series with the supply socket, as D1 is in the reference schematic http://arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf, then reversing the supply will cause no problem: it just won't work until you connect it correctly.
Yes, the diode (if present) will protect against reverse polarity.
I don't know if this helps: Arduino is center +, shell -.
Awesome!!
it worked :), thanks all.
Note that the Vin pin is not protected by that diode, so more care is required if powering
via that pin.