After a bit of a long wait, today I received a response from John Williams.
He signed and personalized the photo I sent, plus signed my Star Wars Trilogy Soundtrack Anthology cover I included as well. My second time getting Williams through the mail. I think the delay was because it was sent to the Boston Pops and not his agent.
Address I used - mailed 08/08/06, rec'd 05/21/07
Mr. John Williams
Boston Symphony Orchestra
Symphony Hall
301 Massachusetts Avenue
Boston, MA 02115
One that someone posted, but I see no successes for yet, is...
c/o The Gorfaine/Schwartz Agency, Inc.
4111 W. Alameda Avenue
Suite 509
Burbank, CA 91505-4140
I'll probably try that one - I saw someone ask him to write some sheet music of a theme of music and he obliged. Sounds neat!