Ha! Finally managed to find her during the day (at least, I'm almost sure this is the same spider). Here she is, resting under a leaf.
In addition to dropping down and pretending to be a piece of bark when I try to photograph her at night, she has the habit of spinning her web in a slightly different spot each night, and she generally destroys most of it by morning. She's camera shy. Silly of her because, as you can see, she's an elegant little beauty with many shades of brown, gold stripes on her leg, and understated striping on the body.
The picture, by the way, is larger than the actual spider. Near as I can tell, she'd about cover the eraser end of a no. 2 pencil with her legs outstretched.
Edit: I was excited to finally have a daylight picture of her and posted it before hearing from the bugguide people. She's almost certainly a Araneus montereyensis. I'm going to keep an eye on her and see what she does.
Right now, she's all curled up under a leaf, legs tucked in, doing her best "I'm just a fleck of detritus" look.