> Everything Steve writes in regard to wiring LEDs is correct, with the 
> exception of this very last phrase. If you wire an LED 
> backwards, nothing 
> much of anything happens. It doesn't glow, but it isn't 
> harmed either. 

It is if the reverse-bias voltage exceeds the breakdown voltage of the
P-N junction: the high current resulting from avalanche multiplication
will cause significant heating; since LEDs are typically not installed
(or even able to be installed) with the heat sink facilities used with
power devices, the device will be destroyed by the resultant high
temperature. I agree that this shouldn't be a problem for pretty much
any LED we're likely to get our hands on; 17.5V is more than enough to
cause breakdown in spherical-junction devices, or in highly-doped
substrates of any junction geometry, but not in other geometry/doping
combinations. I vaguely recall that LEDs fall into the low-medium doping
and planar junction categories, so they should be safe up to a couple of
hundred volts of reverse bias.

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