thermistor controlled luxeons

i was thinking if a ptc thermistor can be used to control the current of a luxeon (or any other make for that matter). thus creating a simple, infinitley self controlling light depending on the heat generated by the LED itself which is the main LED killing factor.
i found these web page
but im not sure which one would be best since there are so many of them and im not that technical.
The ideal would be to put one right inbetween the emitter and the heatsink for better accuracy.

Edit : intended for DD not regulated lights

has this been done before ?

I've looked at it on paper using other PTC devices. I found that the response time is slow for adequate protection if you have a power source that otherwise could damage the emitter and the current tolerances on the devices were fairly broad. The devices you posted a link to insert burdens on the order of 1-2V into the circuit [product of current and resistance of the device]. If I have that much overhead to work with I favor active circuit elements for current control.