I just had a look on t'interweb and it says that in the UK, as well as in other countries, a line polarity reversal takes place to inform caller ID equipment that the CID data burst is the next thing that's going to be placed on the line. Following the data burst, the AC ring voltage is placed on the line, and everything then proceeds as expected for an incoming call. It further said that a line polarity reversal can also be used to signal a remote party call termination, and can be used by some PBX equipment to initiate a cleardown.
So I would expect that there is a distinct possibility that similar signalling is used by at least some carriers in the U.S. , which means that the OP should definitely place his LED across a bridge, and feed the bridge with a suitable value resistor to provide current limiting for the LED, as well as a degree of isolation from the phone network. If he uses a high efficiency LED and a resistor of at least 10k, all the line conditions of idle, looped and ringing will be catered for, and as an added bonus, the voltage level can probably be estimated by eye, as a function of how bright the LED is.
Arfa