Safe Powering Methods - Working with Children
A PoE switch is normally used to power IP phones over an Ethernet cable without having to have an additional power adaptor. It sounds like you're doing a something similar with your devices.

The midspan injector will certainly be a cheap way to add power to an unpowered Ethernet port, with the added advantage of being able to control the supply voltage. You can get PoE switches with everything built in, so I also recommend looking at these as it might be cleaner and easier to use one, but it might cost a bit more.

You could also make your own version of the midspan injector for even less. You'll need some RJ45 sockets and a power supply, and just simply connect the data pins straight through, and the supply to the output sockets to the power pins.

Here is a table of the pinout for RJ45 ethernet with and without power:



One thing you need to be careful of is that 'standard' PoE is 48v, which will drop to as much as 32v depending on the length of your ethernet cable. Make sure that when you split out the supply from the ethernet on the Ardunio end you have something to reduce the voltage to 9-12v. If you're using an injector then you can control this and supply a lower voltage, but if you use a standard PoE switch you need to be aware of this.

What type of sensory devices are you going to be connecting to your PoE ports?