FWIW, I've just finished characterizing the input voltage range and power consumption of the Linksys WAP54G V.2 access point, and posted a graph at
(*) My power supply only goes up to 18.86 volts, and it wasn't worth trying to exceed that. The limit is probably the input capacitor voltage rating, which in this instance is 25V, though I wouldn't depend on that always being true. The supplies Wall Wart (TM) is rated at 12V, 500mA, but it's unregulated and puts out 17.3V open-circuit. Since our local guru Jeff L. has measured WRT54G routers running a bit lower than 4.2 volts, there's probably some margin for error and/or variation between units, temperature effects, etc, so I'd supply them a minimum of 5V.
As an aside, there's plenty of opportunity for Linksys to make a "compact" version of the WAP54G, as the circuit board hiding inside this standard Large Linksys Box is only 3.1"x5.8"x0.75" (plus antennas). Looks like it'd be pretty trivial to add a ground wire and a supply diode between the Ethernet connector and the power connector and feed it power thru the spare pairs. Then the only 'hard' part of the power-over-cat5-cable thing is injecting it at the other end of the cable.