Las Vegas will once again participate in Earth Hour. Most of the hotels on the Strip (and I assume downtown and off-Strip) will be turning off their exterior lights from 8:30pm to 9:30pm on Saturday, March 26th.
I think this is the third year Las Vegas is participating. I was on the Strip for the first one in 2009, but it wasn't widely publicized so I missed it because I wasn't outside at all during the hour.
It's probably an easy decision for Las Vegas hotels to participate. Along with the good PR, they must save a lot on the electricity bill for the night.