Bear canisters are required along a substantial portion of the PCT in the Sierra. Hiking without a canister may result in a hefty fine. For the latest information on canister requirement locations see http://www.sierrawildbear.gov, for a map showing these areas.
Many food storage boxes can be found in bear sensitive areas as well. While you must carry a canister, others have opted to hike from one storage box to the next, storing their food in a box during the night. While not carrying a canister, it is effective. Here is a map showing food storage boxes in the Sierra, on and off the PCT:
http://postholer.com/gmap/gmap.php?trail_id=1&lat=36.985003&lon=-118.520508&zoom=7&markertype=13
Lastly for the civil disobedient, some folks will sleep with their food and defend it should the occassion arise. This is not recommended of course.
A free canister loaner program is in place managed by Jim Payne and the Saufleys. This allows northbound thru-hikers to pick up a cansiter in Agua Dulce, then mail it back when you are done. Further info can be found in the PCT FAQ:
http://postholer.com/faq.php#canLoaner
Considerable information regarding bear cansiters compiled by Jim Payne can be found in the PCT FAQ:
http://postholer.com/faq.php#Canister
The bottom line is to keep food away from bears for their sake. Do what you must to ensure this doesn't happen.
-postholer