Here's an interesting tidbit - some larger data centers are moving to DC power in order to cut down on heat inside the datacenter:
A typical power supply, which converts AC power into the various DC voltages required by individual server components, has an efficiency range of just 65% to 85%, vendors say. Just one 1-kilowatt power supply may generate 300 watts of waste heat, and today's blade servers can consume more than 14 kilowatts per rack.
Some data center managers have responded by using DC-based power distribution systems, eliminating the need for AC power supplies for server racks. IBM and HP both offer servers that can accept bulk DC power from a centralized, telecommunications-grade -48-volt DC power distribution unit (PDU) and then step it down to the voltages required at the server level.
Since no one actually produces DC power anymore (although, it's been produced more recently than I thought - witness this Boston story), these firms are doing the conversion temselves, just outside the data center. That way, the heat dissipation occurs in an area that doesn't need massive AC.