Last week Microsoft's Bill Gates expressed regret about being the world's richest man. When I read this, I was reminded of the expression, "be careful what you wish for, you might get it."

Before launching into an analysis of how Gates could be toppled from his throne, it's worth noting that I've admired Microsoft and wondered whether it's lost its elbow room. I praised Microsoft's ability to adapt to change in two of my books, The Technology Leaders and Value Leadership. But in the last several years, Microsoft seems to have lost its mojo as I noted in these interviews by Red Herring and The Washington Post.

Here's a surprise. The biggest threat to Gates's top rank on the Forbes 400 comes not from the number two on the list, but from numbers 15 and 16. According to that September 2005 list, Gates's net worth totaled $51 billion. Berkshire Hathaway's Warren Buffett came in second at $49 billion. And he was followed by Microsoft co-founder Paul Allen ($22.5B), Dell's Michael Dell ($18B) and Oracle's Larry Ellison ($17B). Spots six through 12 were occupied by descendants of Wal-Mart founder Sam Walton, Microsoft CEO, Steve Ballmer, and heiresses from Cox Enterprises and Fidelity.