I'd suggest getting a poe injector like http://www.dealextreme.com/p/poe-power-over-ethernet-500ma-power-supply-51068Maybe Silicon Dust will do PoE in a future product. I think the limiting factor for PoE is that there aren't many PoE switches available to consumers. I think the cheapest gigabit PoE switch is the Netgear GS110TP (8 port "smart" switch /w PoE) @ ~$260USD.
(note that I don't recommend that specific model). I haved used many of those injector professionnally and I don't expect that they cost much more than 15$ (ordered from China, of course). In any ways, it is much cheaper than a switch.
What you have to watch for however, is how the power is injected. The 802.3af allows 2 ways to inject power. One method is compatible with Gigabit ethernet, the other is not. So when bying a power injecter, make sure that it's written on the power supply that it is gigabit compatible (if you need gigabit, of course).
Another thing to consider is that POE (802.3af) only delivers 12.95W to the device, so if your device uses more than that, you don't have a solution. There is POE2 (802.3at), but I haven't used those device so I can't comment on availability.
A third thing to consider is the effect of the sun. When designing casing for product and figuring out what is the size required to dissipate the power generated by the electronic, we find it is VERY difficult to design for something exposed to the sun. It is increadible how much heat the sun generates on an enclosure. It would most probably required forced ventilation (a fan) which then will allow humidity to get in the enclosure, and require more power (I haven't seen a poe fan...)
ps: I also saw http://www.dealextreme.com/p/poe-power-over-ethernet-power-injector-splitter-combiner-100-240v-37098 which looks fun. It injects power over the ethernet wire and then splits it on the other side to 5V, 7.5V, 9V or 12V. If you have a device that already uses a wall-wart at one of those voltage, then it may be a solution! And you can also find some fans running on DC.