I've installed a few Underfloor heating systems as well as Under-tile systems.
These have invariably been of the electric element type.
Some examples:
Warm Up NZ
And the one I've used a bit:
Devi-Flex

Some pictures of the Devi-flex system, before the concrete slab is poured:

The cold tails and sensor tube:

[Linked Image]

The element wires correctly fitted to the re-bar:

[Linked Image]

John (Reno),Is quite right about the testing required before, during and after installation of this sort of system.
It should also be stated that the main cause of failure of an electric underfloor heating system here in New Zealand, is by the failure of the installing electrician to fit Surge Diverters at the switchboard to protect the UF Heat system.
First lightning strike on the mains nearby, will cause the elements to irrepairably fail.