I'm not sure about that. If it's a stationary, long-term thing, just tap the main power conduit on whatever device you're hacking.
If it's mobile or short-term, Li-Ion cells are pretty awesome. For a few bucks, you could buy a Li-Ion cell, and a smart charger that will switch the load between battery and main power depending on voltage levels.
I'm surprised there isn't more use of Power-over-Ethernet or similar concepts in projects. Ethernet doesn't need complicated configuration or crypto for basic security. It's not a good choice for general consumer products (how many people have cat6 running through their house?) but I imagine that won't slow down people assembling IoT projects.
PoE is on one hand (relatively, for ~$10 devices) expensive to do, with all the signaling and filtering needed, while on the other hand, not abundant in the environments where these projects are run.
I do have an 8-port-1Gb/s-switch with 4-PoE ports at home. It cost 5 times as much as the "plain" 8-port-1Gb/s with no PoE, and I'm the only person I know who has one at home.
Oh definitely. "Real" PoE is expensive and unhelpful but it doesn't have to be standard PoE. You can just steal some pairs from cat5 and still get some form of ethernet. There are even cheap splitters/injectors out there. You won't get the nifty auto-negotiation, filtering, or anything else but it'll work.
In addition, this thing will not ship for anything close to $9 in the near future.
The BLE and WiFi bill of materials is more than that-it's why the BeagleBone Black and RaspberryPi don't include them.