Bishop peak has three summits (east, central and west). Most hikers climb the east summit as it is non-technical. The true highest point of Bishop Peak is the central summit. The "easiest" climb on the summit block of the central summit seems to be rated 5.6 and is a friction climb. This route on the summit block is about 30 feet in length and faces east. As of November 2011, there were four bolts on this face. There were also two bolts on top spaced about two feet apart.
Here's a picture of the summit area courtesy of www.centralcoastclimbing.com:
Note: the climbing guidebook California Central Coast Climbing: San Luis Obispo by Tom Slater covers rock climbs on the central summit of Bishop Peak. I took a cursory look through the section and didn't see the 5.6 climb covered however. Most of the other established climbs to the top of the block were rated 5.10+. On the www.summitpost.com California forum, Bob Burd rated the friction climb class 4. It seem like 5.2 to me. Go climb it and give it your rating!
Bishops Peak Climbing Guide c/o Totaltwerp.com
Rock Climbs of San Luis Obispo's SEVEN SISTERS
By Tom Slater, 1995, 1st edition. 68 pp, topos, photos
Also: additional information on The Nine Sisters of San Luis Obispo County
Also: Mountains of Fire: San Luis Obispo County's Famous Nine Sisters - A Chain of Ancient Volcanic Peaks
by Sharon Lewis Dickerson