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There is already such a protocol. For devices > 14W a second standard would be needed, but it would just be a tweak to the current standard to change wiring/values to something more suitable for large appliances.

http://www.poweroverethernet.com/

It shouldn't be too hard to mod a Mac Mini to use Pover Over Ethernet. I'm not sure if POE provides enough power though. This page claims that about 13 watts is available. Even 15.4 watts would be too low. In the article it says the power usage can spike to 20 watts. You'd probably need a very high efficiency DC-DC converter and you may have to run the Mac Mini diskless or maybe with a flash card instead of a hard drive.

The Power over ethernet website which has links to articles and products (check the dates on some of the articles).

A good article about ins and outs of PoE

Don't confuse PoE with Perl Object Environment or Poe Puzzle

Nevertheless it is a good concept with lot of benefits (as well as drawbacks)

Cost savings. PoE significantly reduces the need for electricians to install conduit, electrical wiring, and outlets throughout the facility. In larger installations, these items can be relatively expensive. Consider an installation of 50 or more access points. This requires lots of conduits, outlet boxes, electrical wiring and the time of a qualified electrician. The low costs of deploying PoE compared to traditional electrical circuits leads to worthwhile returns on investment.

Flexible access point locations. With PoE, a wireless LAN designer has greater freedom to locate access points. You don't need to depend on only locations within short distances from AC outlets. The independence from AC outlets also makes it easier to relocate access points in the future if needed to fine-tune RF coverage or increase capacity. Thus, PoE enables companies to more easily maximize the performance of a wireless LAN.

Higher reliability. Systems with fewer wires tend to be more reliable. With WLANs not using PoE, cleaning people may unplug an access point to use its AC outlet to power vacuum and buffing equipment. Electricians rewiring electrical circuits could inadvertently cut power to an access point. PoE eliminates the possibility of situations that disrupt the operation of the network.

Enhanced operational support. Many PoE devices implement SNMP (simple network management protocol), which enables support staff to remotely manage the electrical power supplied to the access points. For example, support staff can disable a PoE-enabled access point by shutting off its power after detecting a breach of security. The temporary disabling of the access point can protect against an intruder from continuing unauthorized access to corporate systems. Other SNMP-based features enable the monitoring of the condition and consumption of power, which enhances the ability to ensure smooth and efficient network operations.

Simpler international development. For manufacturers, PoE offers the benefit of the vendor not needing to provide different power cords for various countries. This not only helps keep the cost of access points done -- it's one less piece of equipment that installers need to worry about

Slashdot has discussed "Power over Ethernet: IEEE 802.3af" and how the Apple AirPort Base Station supports POE. There also appears to be a website dedicated to Power over Ethernet. Ummmmm ... maybe I could use POE to power my christmas lights ... although I'd need a bit more than 15 Watts! ;-)

I noticed the article didn't link to any images of the USB Razor, but I seem to have found one, rather low res...
here's the link

Also, I found a decent image of the USB Security camera

You've obviously never heard of 802.11af. What's to say that a VoIP telco couldn't replace your "corded phone" with an IP set, also plugged into your wall, but using an RJ45 jack instead of an RJ11?

I realize that standard ethernet doesn't have a VCC line. But you can be more creative: since data is transmitted by a modulated signal, you can use a DC potential between the two pairs to transmit power, or you can use the two spare pairs that are unused by most installations.

See this diagram.

802.3af requires upgraded equipment which is currently rare. Therefore my question: how much load can you put on the rx pair without degrading the signal or hurting the other machine? Probably not very much, but more than zero. Not enough, I suppose, to run a Wifi emitter.

do power over ethernet, then it can be just line a standard phone line and no worries about power outages.