Our friend Tom Lambert has a post up on his blog, YosemiteExplorer.com about the Royal Arches fire; In a nutshell, a fire was spotted on Monday night at that spot. Speculation is that climbers coming off the Royal Arches route lit it to get warm, and didn’t extinguish it properly.
You can read Tom’s account here:
http://yosemiteexplorer.com/yosemite-news/080507-royal-arches-fire
See his images here:
http://yosemiteexplorer.com/albums/v/pow/royalarchesfire/
There’s a discussion on Supertopo here:
http://www.supertopo.com/climbing/thread.html?topic_id=591178&tn=0