In an unusual and personal announcement, Mr. Buffett said the resignation followed revelations that Mr. Sokol had purchased roughly $10 million in shares of a chemicals company that Berkshire recently agreed to buy at the suggestion of Mr. Sokol, Lubrizol Corp.

Mr. Buffett said Mr. Sokol, 54 years old, had bought 96,060 shares in January, before Berkshire reached a $9 billion deal to acquire the company. Berkshire's purchase price of $135 per share meant that Mr. Sokol's stake rose $3 million in value.

Mr. Buffett said he and Mr. Sokol didn't feel the Lubrizol purchases were "in any way unlawful." 

 The SEC is reviewing the Berkshire press release and considering whether to launch an investigation, a person familiar with the matter said Wednesday.

Mr Sokol is, predictibly, going to spend more time with his family, presumably complaining about being fired.

You frequently see problems like this at family owned companies:  the children get tired of waiting for Dad to die, and start freelancing.  The problem is presumably worse at Berkshire Hathaway, because Warren Buffett isn't their father, and he's very long-lived.  I interviewed Bill Gross last week for an upcoming column, and one of the things he said is that investing is very intense work, and Warren Buffett is one of the few people who can keep it up forever: he sits in his office in Omaha, he reads the papers and his annual reports, and if he feels like investing in something he does; otherwise, he just sits on the cash.  He doesn't have to constantly react to volatile markets.

That's ideal for keeping Buffett alive and active for a long time, but it's a big problem for succession planning.  Sure, Buffett's getting older, and he can't last forever.  But at this point, the folks on the short list to succeed him may feel the same way.