This is the spring-flying Martin's Yucca Giant Skipper, Megathymus yuccae martini. Cottonwood Spring, Joshua Tree National Park, March 28, 2010.
Ventral of a different Megathymus yuccae martini, same day as above.
Close view of Martin's Yucca Giant Skipper on the same hike. I saw about 5 at Cottonwood Spring this year; the last couple of spring visits I hadn't seen it at all.