Our favourite search engine uses Opterons to power their datacenters:
http://www.informationweek.com/hardware/showArticle.jhtml?articleID=192300294
Some of the new servers going into Google's data centers are probably equipped with AMD Opteron multicore processors. Google won't confirm that, but one of the reasons AMD chips are selling so well to other companies is that they don't throw off as much heat as older alternatives. Google engineers, who pay close attention to microprocessor efficiency and heat dissipation, must find AMD chips hard to ignore. Intel is racing to improve the performance-per-watt of its own processor line.
http://www.informationweek.com/hardware/showArticle.jhtml?articleID=192300294