It’s D-Day for the World’s Greatest Investor.

Starting at around 9:30 this morning, Warren Buffett will stand on the stage of the cavernous Qwest Center in Omaha and face the music. Three journalists will ask him questions culled from thousands that have been e-mailed by shareholders of Berkshire Hathaway, Buffett’s sprawling holding company. The audience will consist of 35,000 of those shareholders who traveled to Omaha to hear him.

Buffett describes this event — a k a the Berkshire Hathaway annual meeting — as “Woodstock for capitalism.” It’s normally a festive affair, with shareholders celebrating a stock that has made many of them wealthy, while Buffett, a shameless ham, cracks wise and dispenses folksy investing wisdom.

This year, though, the tone is likely to be more somber, thanks to l’affaire Sokol, which has seriously dented Buffett’s pristine image. For someone who has said repeatedly that he would rather lose money than even a shred of reputation, his actions have been inexplicable. Thus the questions this morning are going to be unusually tough. As they should be.

David Sokol, of course, was the trusted Buffett aide who left Berkshire last month after it was revealed that he had bought $10 million worth of Lubrizol, a company he then convinced Buffett to buy — giving Sokol a nifty $3 million profit in a few weeks’ time. To his credit, Buffett is the one who revealed this information in a press release.