First of all - DON'T DO IT.
However, IF you must pull it out you have to wait five years and then can only pull out contributions - not gains.
My husband started his Roth in 1997 with $8400 (rolled over his former employers 401k balance and paid income tax on it).
Fast forward to 2010. We bought a house and w/drew $8000 for the down payment. I REALLY regret that decision.
On the bright it has continued to gain, pay dividends, and we'll max out contributions to it for the 3rd year straight next month.
Current value = $40,000.
Bottom line if can find another way to access cash - do it - don't take $$$ from your Roth!!!