I'll participate in EarthHour for one simple reason: light pollution. I mostly call this day, "Take advantage of environmentalists for slightly darker astronomical observation day."

Sure its a symbolic gesture and it could be argued that it really doesn't amount to anything (and it leads to no participation on a basis of that principle), but if it gets a large enough faction of people to dim the orange "bleh" that is my night time sky, I'm okay with it. It's a good enough excuse for me to chill under a slightly less orange sky with my telescope at hand that's been eating dust for a good year.

Reminds me of that one Simpson episode where Lisa convinces the town to shut off the power to the city.