A research report says that Linux boxes get used for twice as long as Windows boxes (just think of all those PCs that'll be thrown out in favor of something fast enough to run Vista's crippleware!). That means that GNU/Linux machines save landfill sites!
"A typical hardware refresh period for Microsoft Windows is 3-4 years. A major UK manufacturing organization quotes its hardware refresh period for Linux systems as 6-8 years." A significant difference...a doubling even, of the lifetime of a computer.
Thus, a world using Linux would be a world with half the computer waste (and, admittedly, halved sales for Dell and the rest.)
A widespread switch to Linux could prevent millions of tons of waste from going into landfills. Every computer not needed would prevent the use of 240 kg of fossil fuels. Spread that out over the 17.5 million computers that wouldn't be going obsolete every year and Linux could deliver the world a much more sustainable future. Link (via Digg)
