Just how much can one activist investor take? A lot, if your name is Bill Ackman.

Ackman's Pershing Square Capital Management does not quietly sit by as the companies it invests in go about their business. Each investment requires a different strategy: When he bought shares of McDonald's two years ago because he thought they were undervalued, Ackman reportedly spent a day behind the counter at a local franchise to see how the hamburger chain works.

When he first noticed weaknesses in the business model of the bond insurer MBIA five years ago, Ackman shorted the stock and promptly started sharing his analysis on the company's impending doom with just about anyone who would listen.

Sometimes the strategy pays off. Ackman sold his stake in McDonald's last week after seeing its shares nearly double.

As for MBIA, Ackman is still sharing.

Long before the subprime crisis emerged, Ackman had been forecasting disaster for MBIA and its chief rival, Ambac Financial Group, which are the holding companies for the two biggest bond insurers. The companies guarantee debt, and Ackman believes that the insurers are in no shape to their handle commitments should the debt they back go bad.

As the write-down party on Wall Street continues full swing, it looks like Ackman may have a point. In fact, he's so sure of himself that he promised to give the profits he makes from his investments in the bond insurers, which he predicts will be hundreds of millions of dollars, to charity.

Ackman must have felt some vindication last week, when Moody's suggested that MBIA's triple-A credit rating was threatened by the fact that the bond insurer most likely had insufficient capital to handle the risk. Shares in MBIA plummeted 16 percent.

That news brought MBIA shares down 62 percent for the year, which is money in the bank for a short seller. But it wasn't good enough for Ackman, who believes that MBIA and Ambac will go bankrupt. After all, a $30 stock, even if it has fallen from $76, still has a long way to go.

And the MBIA investor stampede came at an opportune time for Ackman, whose portfolio had another holding in trouble. Just days before the Moody's announcement, shares in the troubled retailer Sears fell 11 percent after it announced that its profit had plunged 99 percent. Ackman had scooped up 5 million shares just a month before.

But activist investing requires an iron stomach—an unflappability of monumental proportions. And Ackman's must have come in handy yesterday, when MBIA announced that the private equity firm Warburg Pincus had agreed to invest as much as $1 billion to help the company boost its capital and maintain its triple-A rating. MBIA shares soared 13 percent, precisely the wrong direction for Pershing.

As for Ackman, he continues sharing, as any dutiful short seller should.

"I generally think that Warburg Pincus is a very smart private equity firm, but I don't think they understand what they just bought," he told the Wall Street Journal. "It's likely that they'll lose their entire investment."

Is someone listening? Shares of MBIA are down 3 percent today, and Ambac is down 5 percent.

All in a day's work.