I couldn't find a thread on this from yesterday but I was flipping around most of the day and Wolf Blitzer asked the White House reporter about when Bush ordered the flags on all federal buildings to be lowered to half mast and she said she thought it was late Saturday or early Sunday, Wolf asked her to check on it. During "the Situation Room" (high comedy in a time of such tragic events) Wolf actually followed up on it.
He reported that the flags were ordered to half staff after Rehnquist's death was announced not for the Katrina victims.
http://www-cgi.cnn.com/2005/US/09/04/katrina.impact/President Bush on Sunday ordered flags at government buildings and military installations across the country to fly at half-staff to mark the death of Chief Justice William Rehnquist and in memory of Hurricane Katrina's victims. (Rehnquist dies)
http://www.dailykos.com/story/2005/9/4/222814/2914