|MadSci Network: Physics|
I found the FAA page you describe, but there's not a lot of information there about this device. I tried web searching on things like 'optical ice sensor' and didn't find anything promising. Then it occurred to me to check the US Patent Office.
There is a patent (number 5760711) for an optical ice sensing device for aircraft; it was granted in June of 1998, so it might be the thing the FAA page refers to. Remember - just because a patent is granted doesn't mean the invention actually works! - but the idea seems to involve firing light pulses at a sensor mounted on the aircraft. If there is a buildup of ice on the sensor more of the light will be reflected into a light detector.
Does it have any limitations? Well, I'm not an an aircraft engineer, but it looks like a pretty rugged system to me. Obviously the fact that the sensor is mounted on the outside of the aircraft means it could be torn off, but I would think that's not an unreasonable limitation. But as I say, I'm not really a good judge - you can read the description for yourself and see what you think.
Again, I don't know if this is the device the FAA document referred to - you best bet would be to ask someone at the FAA directly. But reading the patent description of this device was interesting anyway!
Try the links in the MadSci Library for more information on Physics.