how do you dump the priv key ?
Go to the debugconsole and enter:
walletpassphrase $walletPassphrase 600

dumpprivkey $address
walletlock
where $walletPassphrase and $address are the ones you refer to.
But beware, your private key is the most important thing about your wallet. Never store it unencrypted anywhere or better never store it digitally. I'm pretty sure you know that, but it's too important no to be mentioned.