If you want to power anything from a 802.3af or at POE switch, it takes more than an electrical connection to do it.
The POE switch port provides 48V at a very low current at the connection time, and there is a negotiation sequence between the device and the switch to enable additional current.
The arduino POE Ethernet shield uses a SilverTone module to do the negotiation and provide a switch mode controller to reduce the POE line voltage to (I think) 12V which is then regulated using the on-board LDO for 5V.
http://www.tme.eu/en/Document/0d7d047fa4d1a6ccc6b0881ecce59981/x000002.pdfThere are other SilverTel modules that provide 5V or 3.3V (at a lower total power output).
Designing a POE device controller from scratch is non-trivial, if you are really interesting in doing this, drop another note and I will provide some links to chipsets you might consider. The Silvertel modules are going to be a better idea unless you are planning on making 100's or 1000's of a device (and maybe even still at that quantity).
Mouser:
http://www.mouser.com/ProductDetail/Arduino/X000002/?qs=sGAEpiMZZMu5STvjs3TtKtwmrcRu2yPiDigikey:
http://www.digikey.com/product-search/en/prog...lopment-systems/accessories/2621524?k=arduino poe
