HP announced today that they will be offering servers in the middle of next year that use ARM chips (and eventually, Intel's Atom processor). The stated benefit is primarily energy efficiency, and HP figures that maybe fifty of the largest data center operators (those with web serving applications) will be interested in the product.
Note the description of the energy use of HP's system compared to a "base" load of 400 servers - energy use drops by almost a factor of ten, from a load of over 90 kW to just over 9 kW. The space required goes from ten racks to just half a rack, so the new architecture pushes the density envelop into the 20 kW per rack range, which can be cooled with air, but is likely to push operators to consider at least close-coupled cooling solutions.
It will be interesting to see if the product line makes headway, and whether other users may adopt it as well. HP notes that scientific and financial processing loads, which require intensive data analysis, might be suitable for the equipment also.
And it appears that we have two fundamentally different approaches to server energy efficiency coming from HP and Dell, with Dell offering more traditional architectures but warranting equipment to much wider environmental conditions - essentially offering equipment that can allow air-side economizer use all of the time, anywhere in the country. HP is countering with fundamentally more efficient equipment, but with the potential liability of high energy density.