Can a thunderbolt cable transmit power?
According to Apple, yes, it can carry power for devices in some circumstances. Specifically:


  7. Does the Thunderbolt port supply power to connected devices?
  
  Yes. A Thunderbolt port supplies power if that Thunderbolt port is part of a device that is connected to an AC power outlet or is operating on battery power. If your Thunderbolt devices have their own power supplies, you should connect the power supplies to them for optimal functionality. If you are using Thunderbolt devices that get their power only from the Thunderbolt bus, computers running on battery power will experience battery drain more quickly.


The Thunderbolt specs on the Wikipedia page for Thunderbolt currently say the copper-based version of the Thunderbolt protocol can deliver 18 V at 550 mA (9.9 W max). Presumably all Thunderbolt cables are built to this spec and can handle that kind of power passing through them. Whether the connected device can supply that kind of power is another question that can only be answered by looking at specific devices.

Thunderbolt over an optical, instead of copper, connection would not be able to carry any power with the data signal. I note this only because Thunderbolt was original designed to be an optical protocol; though its since been adapted to copper which is a good thing.