Apple Computers now has a very nice website that shows the key life cycle impacts of all current products. The 20-inch iMac, for example, generates 870 Kg of CO2e over its life cycle, with 44% coming from production, 50% from usage (over a 4-year use phase), and the rest from transport and recycling. Company-wide, 38% of product carbon emissions are from production (this includes extraction of raw materials and assembly) and 53% from the use phase. The computer weighs just 8.3 Kg and the enclosure is highly recyclable.
This clearly shows the use phase contributing less to the carbon footprint compared to previous generation computers (I remember an older estimate of around 80% for the use phase), a lot of it from smarter power management. Shrinking the production emissions may be the more difficult task.
