Assuming that there are significant synergy gains (multiple transmitter close by resulting in greater efficiency), one could use centimeter or inch scale devices. With a roughly even distribution, 10 to 100 of these devices in a room, or 3 to 30 per person (embedding several in each piece of clothing, plus cell phone, headset, mp3 player and pda makes this believable). Any smaller will run into the microwave range, and a prohibitively high number of units. Still, multiple devices per person, as well as 10’s of devices in an unoccupied room is an extremely high cost for even just city level coverage. (New York would have an easy time, just a hide a standard transmitter and receiver in all new cell phones. 10-100 transmit only devices per block, or 20 to a story for buildings over 3 stories, and instant city-wide mesh-grid in less than five years). Lower population densities would need more repeaters, or much bigger repeaters. Where I live, making a device the size of my house (embed a loop in the walls that circumscribes the house) would be insufficient to link to any of my neighbors, and only other house sized devices could be linked to by this device.

All of this requires government level standardization; truly pervasive levels of use would require potentially campaign breaking levels of funding across multiple administrations. Hybrid tech (new electrical outlets, lights, and cell phones with embedded WiTri), would be the best route, but it would likely require either mandates or subsidies to be effective.

The craziest solution would be a WiFi/WiTri or a BlueTooth/WiTri repeater built into fluorescent (or LED) light bulbs. Current LiIon batteries wear out at the same rate as cheap fluorescent light bulbs, and wireless tech is sufficiently volatile to require replacement every few years at the minimum.
Synergy effects come into play with hard wired, battery-backed, spatially distributed micro-controllers. (Can we say multiply redundant security, climate control, and system health 24-7 monitor, as well a built in computation-grid and emergency power source, that is dutifully maintained every two years just by replacing burned out light bulbs, all for the cost of lights, wireless network, and emergency power hardware.