Whether or not you think the rich pay their fair share is kind of immaterial to the scale of the budget problems we now face. The combined net worth of the Forbes 400 is estimated at $1.37 trillion. The budget currently under discussion for fiscal year 2011 has a deficit of about $1.5 trillion, according to the CBO. So assuming you could tax 100% of the wealth of the Forbes 400 and apply it to the deficit, you would still be $130 billion dollars short.
That's not the point. And no one is suggesting taxing 100% of their wealth. What's being suggested is closing the deficit through a combination of A) increasing taxes on those most able to pay (by eliminating loopholes and then, if necessary, increasing the rate), and B) cutting spending that isn't actually benefiting the American public as a whole (as opposed to special interests and the well-connected elite). In other words, you're increasing your revenue and decreasing your expenses
simultaneously in order to bring the two into balance; you have to do both if you expect to close the deficit any time soon.
But that's the short-term fix; over the long term, the budget needs to be reprioritized to cut out all of the non-core spending and focus on providing the things that government does best - public goods. Once that's done, taxes can be adjusted downward from there.