The green states are officially participating in Earth Hour.
It is hard to find out exactly what is happening on Earth Hour; most activities are pretty local. Thirty states are turning out the lights in their governor's mansions and other public buildings, and according to USA Today, the Empire State Building, the Golden Gate Bridge, the Las Vegas Strip, Niagara Falls and the Willis Tower, formerly the Sears Tower, are all shutting them down. (although why office buildings like the Empire State and the Sears tower would have them on during a Saturday night is beyond me)
Even Mount Rushmore is shutting off the lights for the first time. "As stewards of our national parks, we want to be visible leaders," says Navnit Singh, spokesman of South Dakota's Mount Rushmore National Memorial.
In Canada, there is a published list of events, and in Australia there is an Earth Hour Blog listing events, but there does not appear to be any registry on the American or UK sites. (If anyone finds any other lists, please post them in comments) They are both suggesting that you look up Earth Hour Events on Facebook.
So you are pretty much on your own; in the end, Earth Hour is a local celebration. Here are some ideas: