I have been thinking about mesh networks recently. Two of the better-known projects in this space are Open-Mesh and the Mesh Potato . Both use 802.11 to build a wireless network, but experience shows that wireless interference is a major problem in scaling these networks.
The solution seems obvious: add a HomePlug power line communications interface to the mesh routers. That way routers adjacent on the power grid (and so presumably adjacent to each other and other wireless devices, that is, in places with a lot of interference) can do forwarding over the wire.
If you add HomePlug interfaces to devices themselves (are there any PC or notebook power supplies with HomePlug built-in?), pervasive mesh networking starts to look like an inevitability.
On further research, I found out about Insteon, a low-bandwidth home automation protocol that employs mesh networking over power lines and 900MHz RF.