Bill Gates and Paul Allen, co-founder of Microsoft, have filed forms to sell small but intensely valuable slices of their shares in the company. The US Securities and Exchange Commission (SEC) requires that insiders file notification of their intention to sell, although they don't actually have to complete the deal. Gates will sell up to 2.5 million shares for something in the region of $250 million, while Allen may shift just over 1 million. In Gates' case he says he's diversifying his portfolio, but as he currently owns around 270 million Microsoft shares he has a way to go. Allen on the other hand probably has more immediate reasons for raising cash. He's been building a portfolio of cable TV assets in the US in anticipation of the digital revolution penetrating consumer markets, and financing network upgrades and set-top box installations could easily swallow a fe billion dollars.