According to Gartner, by 2012, PCs will be so energy efficient that 60 percent of the greenhouse gasses attributed to them will have been generated before it’s ever turned on.

From eWeek:

Regarding environmental breakthroughs, by 2012, Gartner expects that 60 percent of a new PC’s total life greenhouse gas emissions will have occurred before the user turns on the machine for the first time, thanks to advancement in PC energy use. Additionally, carbon remediation costs are expected to be included in most IT business cases by 2014.

Source: eWeek