This is a Western Pine Elfin (Incisalia eryphon). In A Field Guide to Butterflies of the San Francisco Bay and Sacramento Valley Regions, Art Shapiro and Tim Manolis describe the pattern on the underside of the hindwing as "reminiscent of a fine Oriental rug."
It's too bad these butterflies are on the small side — with a wingspan of about 1 inch (2.5 cm) across. Otherwise, I think they'd get a lot more attention. Next time you see a brownish butterfly land near a pine, you might want to take a closer look!
Western Pine Elfins are spring fliers. Watch for them between March and June.