When designing our Ethernet Shield one of the most important aspects was support for Power-over-Ethernet. I hate having to run multiple cables to Arduino-based network nodes in my home automation system, so having support for PoE was right at the top of the feature list.
PoE can be achieved the cheap DIY way, or it can be done with commercial PoE switches or midspan injectors. Because not everyone wants to use PoE we didn't include it in the basic Ethernet shield but instead brought the necessary connections up on top to a PoE header, allowing use of daughter-boards to support different types of PoE.
Now, thanks to our brand new Power-over-Ethernet Regulator 802.3af, the Ethernet Shield can operate with commercial PoE systems running at a nominal 48V. The regulator daughter-board mounts on the Ethernet Shield and implements the signalling mechanism and voltage regulation necessary to drop the 48V supplied on the wire down to 7.5V to be fed into the Arduino's onboard voltage regulator.
Because it uses a switch-mode voltage regulator it runs nice and cool - important for devices embedded in unusual places such as inside walls and ceilings!
For more background information see our tutorial "Power-over-Ethernet for Arduino".