I love the idea of these, its just the cost !
A company is Japan is marketing a pot which includes thermoelectric cells specifically for phone charging (post tsunami).
http://iphonedownloadworld.com/06/japanese-company-invents-hot-pot-phone-charger-just-add-fire/(no affiliations etc)
Although the cost is quite high for a finished product (IMO), the thermoelectric cells themselves are relatively inexpensive and can be purchased quite readily.
I have toyed with the idea building something myself ... perhaps one day ! The thing to be wary of (if purchasing) is that all the thermoelectric cells have differnt characteristics and work best at different temperature ranges and produce different outputs, so the idea of using one in boiling water is good because the temperature is managed by the boiling point of water itself.
There is lots you can read online about it.
Thermoelectric cells can be used to "recover" waste heat from industrial settings, but because they are considered inefficient, they don't seem to be widely used. However in a camp site setting we are not too worried about "efficiency", so its a good application. IMO, also good if you have a wood heater at home.