"Hey, Jenna Bush! Your mother's so fat she's got her own zip code!"
... and run off laughing.
20500 is the Washington, D.C. zip code that is reserved for the use of the U.S. President and First Family. Therefore, to write to George W., address your letter to:
President George W. Bush
The White House, 1600 Pennsylvania Ave.
Washington, D.C. 20500
The zip code was created to account for the vast quantity of letters that are written to the President every day. Sort of like the holding area for children's letters to Santa Claus, except less of them get through.
Similar Zip Codes:
20510 - The Senate
20515 - The House of Representatives
20530 - The Cabinet
20543 - The Supreme Court