I don't think it's a 10pin jack on the "ethernet" side, but in order to implement the Power-over-Ethernet capability, the connector has to provide both a filtered signal output ("with magnetics") as well as a "raw" (rawer, anyway) output for the power. So it has more pins coming out than it has going in. Silvertel (who makes the PoE modual) has a list of compatible jacks here: http://www.silvertel.com/poe_magnetics.htm
I'm not sure which one the Arduino shield actually uses.