I believe the ethernet jack on newer receivers are for the purpose of making the unit part of a "digital home entertainment" setup. In other words, you can play mp3 files stored on a computer or play streaming internet radio directly through your receiver. In other words, straight data.
Coax is used to send either digital (data) or analog signals between components. It acts as a point-to-point interconnect. Fiber does the same, but is digital only. I don't see Cat-5/5e becoming an alernate standard for interconnects per se, but I do see them becoming a standard feature on more and more devices.
Let's say as time goes on more devices are embedded with chips that can decode digital audio signals. Say your CD player has ethernet capability. You set it to bypass its DAC and send a compressed digital stream onto your network. Now any device connected to your network can pick up the stream, decode it, and play it. So you could have little "mini-receivers" all over the house, in the back yard, etc. Or even more likely, powered speakers that take the signals and play them back directly. My guess is you will start to see wireless ethernet antennas sticking out of receivers in the near future as well.
