It may help others to know that I tracked down the source of the
problem to the Vacuum Switching Valve for the Pressure Sensor
mounted on the canister. It was stuck open.  I took it apart and
found the little piston had rust on it.  (had to force it out with
compressed air).  I sanded the rust, put it back together and it
works fine.
This is located just behind the gas tank in a awful location to
access (not one of Toyota's better ideas).  It has to be a
vulnerable part, and a likely suspect when the P0446 code appears.  
The good news is that they can be taken apart and cleaned saving $.
My Camry now has 163,000 miles on the clock, and this is the first
problem I've had.  What a car!
Fred



Thanks for posting your fix. Wish more people did. Had this problem
myself several years ago & luckily was still under warranty but
wouldn't be surprised to see it surface again. Once again, thanks.
dc