by looking at Greg's Chipguide.com. He lists which casinos closed...and which were reopened, etc and marks them Katrina. Skip around the MS and LA casinos and you can see the dates on the top left.

http://www.chipguide.com/cgi-bin/search/search.cgi?id=mississi

http://www.chipguide.com/cgi-bin/search/search.cgi?id=louis

The vast majority of the casinos effected are (in MS) Biloxi, Gulfport, Bay St. Louis and of course New Orleans in LA.