That's interesting. It also means that, if you look at the numbers, even a shut-down government runs a deficit and spends more than it takes in. According to Wikipedia, the current planned budget is 3.8 trillion, and expected income will be 2.9 trillion, for a deficit of 900 billion.
But, if 17% of the government is really shut down (and that may be high, because of votes to guarantee back pay to "furloughed" non-essential employees), the spending cut would be roughly $600 billion, so we'd still spend at a $3.2 trillion rate, leaving a deficit between $200 and $300 billion!
Good point. Once back-pay is inserted into the equation I would think the savings in a shudown is very low. A shutdown just means we pay government employees to goof off and harrass us at monuments.