Since LED lights aren't generally available yet, the dimmer manufacturers are mostly justified in that statement! :-)

LEDs are low voltage devices, sure. So they'll use multiple LEDs in series to make the voltage up, or use a transformer to drop the voltage.

The behaviour of a LED-plus-resistor combination is that it does nothing at lower voltages, then it starts conducting and lighting. With the resistor in there, as the voltage rises above that break-point, the light level increases. That means there'll always be a portion of the sine-wave where the LEDs do nothing. When the steps in your dimmer coincide with this region, the dimmer won't have any effect, because the LEDs won't light anyway. But the steps in the dimmer which *do* coincide with the LED being on will dim the LED as normal. So as I said, the worst-case is that the dimmer steps aren't the same as before.

This is assuming they don't try anything fancy with the LEDs. It would be easily possible to make up a circuit which compensates for voltage fluctuations and so on by converting the AC to DC and regulating it- in that case, a dimmer wouldn't work. This circuit would be less efficient though and for no real gain, so I can't see this happening when LEDs get into mainstream use.

Graham.