I also saw a pair of RJ45 plug/sockets pairs with 2.1mm Power plug or socket (one of each) as CPC (at http://cpc.farnell.com )Website terms prevents providing direct item link! with a part number SR09618 (CPC is the Consumer division of Farnell, one of the Official UK RPi suppliers) - at the time of writing they were on offer for £5.33 + V.A.T. under the extended code SR0961887. These would enable you to couple power into and out of a Cat5+ cable (the instructions do call for copper-cored wires in that cable NOT copper-cased-aluminium ones) in the same way as part of yingw's answer does using some home-made adapters.
I think you will need to use special POE hubs/switches if you want a more extensive system than routing Power over more than just one cable.
You will also want to account for any voltage drop along the cable - perhaps by injecting 7 Vdc in at the source end and use a local regulator to provide the exact 5 Volt, 1 to 2 Amp supply for the Pi. (Or by using a buck/boost regulator module from a supplier on an on-line auction site, they can be sourced for only a few pounds...)