Check this entry I just found on wikipedia. I've been seeing flags at half staff today and I was wondering why. None of the news noted any unexpected tragedies in the world. I'm guessing it was to mark the anniversary of Hurricane Katrina. Anyway, i found the flag flying rules:

"United States

In the United States, the President or Governor of a state or territory can issue an Executive Order for the flag of the United States to be flown at half-staff upon the death of principal figures of the United States government, and others, as a mark of respect to their memory. When such an order is issued, government buildings, offices, and military bases are to fly their flags at half-staff. Since it is a generally accepted practice to fly the US flag higher than all other flags private organizations and individuals usually also fly their flags at half-staff at these times, but are not required by law to do so. Governors, by law and custom of their particular state, are able to direct by Executive Order their respective state flags be flown at half-staff.

Upon the order of the President, the flag is to be flown at half-staff in following circumstances:

• For thirty days after the death of a current or former President.
• For ten days after the death of a current or former Vice-President, Chief Justice, or Speaker of the House of Representatives.
• From the day of death until interment of an Associate Justice of the Supreme Court, a Secretary of an executive or military department, a former Vice President, or the Governor of a State, territory, or possession
• On the day of death and the following day for a Member of Congress
• On Memorial Day until noon
• Annually by presidential proclamation, (as requested by U.S. Congress) on September 11 (Patriot Day), and December 7, National Pearl Harbor Remembrance Day, (see US Code: Title 36, Chapter 1)
• Upon presidential proclamation, which have included: the remembrance of the 9/11 attacks, the death of Pope John Paul II, the Space Shuttle Columbia disaster in 2003, the victims of Hurricane Katrina, and the Indian Ocean tsunami, death of Coretta Scott King

US federal law requires the flag be flown at half-mast on Peace Officers Memorial Day (May 15), unless that day is also Armed Forces Day. Yearly presidential proclamations also mandate that the flag be flown at half-staff on Pearl Harbor Remembrance Day (December 7) and National Korean War Veterans Armistice Day (July 27)."

(my emphasis).

These rules have been nearly unchanged for over 200 years.
Just saying...