Just wondering if you would be better off with some sort of Xbee or radio network. The sensors you linked don't have ethernet so you will be having something like 17 x $80 for the ethernet boards plus 17 x $12 for the sensors. That's $1564 already.
Those sensors seem to have some sort of serial output, so you might be able to hook a few up to a single processor (eg. all the ones in one room).
I've figured that I'd like to (ideally) have about 17 or so PIR sensors in locations at the most 15-20 meters from the Arduino (most would be closer).
Or are you talking about a single Arduino? In which case it would need 17 serial inputs.
If the sensors output a simple yes/no line, then you could probably do that fairly easily. For example, a 16-port port expander could simplify things.
eg. Gammon Forum : Electronics : Microprocessors : Buttons, I2C, interrupts and port-expanders
Would category 6 Ethernet be an appropriate wire to use from the Arduino to each PIR sensor?
Sounds like an overkill, especially if it just raises an on/off signal. I would be inclined to get a couple, a reel of wire, and experiment. However if interference is an issue you could turn the signal into a balanced one, eg.
My head is swimming in the possibilities but I haven't committed to anything yet.
My advice is to get an Arduino (or two), get some sensors, some wire, and experiment. Your experiments with wire slung over the floor in a real-life situation will reveal more than us forum guys speculating.