Seagate and Hitachi GST are promoting the idea of having 2.5 inch drives packed into drive arrays as a way of increasing I/O speed. Let's assume a 10 TB array of 500GB 3.5 inch drives and a 10TB array of 250GB 2.5 inch drives, 40 drives versus the 3.5 inch array's 20 drives. Effectively you have many more read heads accessing the data in the 2.5 inch drive array.
This sounds good until you look at the power costs. You have twice as many electric motors in the 2.5 inch drive array which means more cooling power is needed. So the energy consumption cost of the 2.5 inch drive array will be much more (2X, 2.5X?) than the 3.5 inch drive array.
This might well rule out 2.5 inch drive arrays from general data centre use. The logic might go 'put fast access files in a RAM-heavy server (Acopia's tier zero) and ignore 2.5 inch array technology completely.'