Update: According to the OC Register, the Irvine fire was set by Arson:
A wildfire that has swept through 8,800 acres of dry wilderness near Irvine was intentionally set in at least three places, firefighters announced this morning as they prepared for another fierce day of wind and heat.
The Orange County Fire Authority listed the cause of the fire as arson after firefighters found three ignition points near Santiago Canyon Road, east of Lake Irvine. No other details were immediately available; Chief Chip Prather made the announcement after he was handed a note during an early-morning news conference.
He asked that anyone with information call a tip line at 800-540-8282.
OC Register also has some good video on the topic. From what I understand, things have calmed down in Irvine, but the fire is still going and soot and is falling.
Update: Flickr has more pictures from users posted to the site.