That's not quite how it works (at least, with most of the online forex trading houses like FXCM). At any given time, they know exactly how much you are up or down in total (ie, including your leverage). If you hit the point where you're going to lose more than what you have in your account, they will perform a margin call and close out all your positions.

The really sucky bit here is (I've had that happen) when the market then immediately turns - you simply couldn't buffer the immediate losses ;-)