A barely visible contrail. It looks at first glance like a single trail, but there are actually two:
The plane is flying south, and is lit from the east. Is the "invisible" contrail in the shadow of the other one? The sun was still relatively low (about 32° according to http://www.esrl.noaa.gov/gmd/grad/solcalc/ ). Pic was taken at 8:40AM, Shingle Springs, California.
It was an A320 at 35,000 feet:
Full set of pics: