From the Wall Street Journal's editorial on the president's budget speech comes this fact: "According to Internal Revenue Service data, the entire taxable income of everyone earning over $100,000 in 2008 was about $1.582 trillion. Even if all these Americans—most of whom are far from wealthy—were taxed at 100%, it wouldn't cover Mr. Obama's deficit for this year."
Update: Commenter Ben points out that 2008 was an unusual year in which people had a lot of losses. I went looking for this data on the IRS Web site (the Journal editorial has no hyperlinks, disappointingly) and found numbers for both 2008 and 2007, which was more of a boom year. By my calculations from those spreadsheets total taxable income of filers above $100,000 was $3.76 trillion in 2007 and $3.4 trillion in 2008, of which those earners were already paying $862 billion in income tax before credits in 2007 and $797 billion in income tax before credits in 2008. Even if you believe my numbers rather than the Journal's and follow Ben's suggestion of using 2007 rather than 2008, that leaves roughly $2.9 trillion in untaxed income ($3.76 trillion minus $862 billion) left over. Some of that money is going to payroll taxes for Medicare and Social Security and to state and local income and property taxes, so it's not available for the federal government to raise taxes on even if it wanted to. So to close this year's federal budget deficit of $1.6 trillion or so just by taxing "the rich" — even using an expansive definition of rich as any tax filer with an annual household income above $100,000 — the government really would need to impose a tax rate up there around 100%. And even that wouldn't work, because why would people invest the effort or take the risk to earn trillions of dollars in income if they knew the government was just going to take it all away from them in taxes? They'd just earn less and enjoy more leisure time, or find tax shelters like offshore accounts or tax-exempt federal and municipal bonds.