In "Power, Pollution and the Internet", JAMES GLANZ (New York Times, September 22, 2012) reports that data centres have low efficiency and high pollution levels due to the need to be ready to respond to peak loads. It is true that data centres use a lot of power, but some centre operators, such as Google (mentioned in the article), are making efforts to run more efficient centres.
Canberra Data Centres, which is used by several government agencies, has implemented several power saving features.
Also the "cloud" server approach with a large data centre can be more efficient than having many smaller servers spread out in companies. The servers can share a load, whereas a small server dedicated to one task will spend most of its time running at near full power doing almost nothing.
As I commended recently at a digital record-keeping conference in Canberra, "cloud" computing has similarities to bureau computing systems used decades ago. There are efficiency measures which can be used to reduce the load on the system and to balance it.
For more on how to measure and reduce the environmental impacts my book "ICT Sustainability: Assessment and Strategies for a Low Carbon Future".