Yes, it's annoying that the official spec is difficult to implement but I can understand why it was done that way. At the time it was drafted it was pretty common for rack-mount gear (particularly telco gear) to run on a 48V power buss with a bank of batteries in series for 48V of backup power, so 48V for PoE makes some kind of weird sense. The higher supply voltage also helps with getting a decent amount of power through a cable with less problems caused by voltage drop.
The signalling system isn't really particularly hard to implement for a commercial device either, and the minimum requirement if you don't want your device to do anything other than indicate the default power rating can be as simple as a single resistor across the appropriate pairs. Just figuring that out from the spec though can be quite an exercise!
One good thing is that the DIY efforts I've seen around the place generally seem to converge on 12V, to the point that there are really two standards: the 48V 802.3af/at "official" standard, and the unofficial de-facto standard of 12V with DC+ on 4/5 and DC- on 7/8. Just about every person I've spoken to about their DIY PoE system has referred to this page at some point:http://www.nycwireless.net/projects/poe-power-over-ethernet/
That page has been hugely influential in pushing people to some commonality in their non-spec PoE systems. What I did with the PoE injector was just a simple, no-soldering-required way to implement what is documented on that page and is already by far the most common way to DIY it.
Practical Arduino: www.practicalarduino.com