The winner for "Most bizarre rumor to come out of Hurricane Katrina" so far is the above-cited Observer report that a group of killer dolphins armed with "toxic dart" guns, and trained by the U.S. Navy to track down and immobilize spies and terrorists, was washed out to sea in the Gulf of Mexico by the storm. The cetaceans are now missing and presumed to be armed and dangerous, at least according to rumor.
The Observer article quotes a "respected accident investigator," Leo Sheridan, who maintains that the killer dolphins now supposedly on the loose pose a serious threat to innocent lives:
My concern is that they have learnt to shoot at divers in wetsuits who have simulated terrorists in exercises. If divers or windsurfers are mistaken for a spy or suicide bomber and if equipped with special harnesses carrying toxic darts, they could fire,' he said. 'The darts are designed to put the target to sleep so they can be interrogated later, but what happens if the victim is not found for hours?'
The sole source for this story seems to be the above-mentioned Mr. Sheridan, whose validity as an information source is dubious at best.
In 2003 Sheridan reported that he believed his private team of divers had located the airplane flown by Amy Taylor (an aviation pioneer who was killed when Airspeed Oxford she was flying from Scotland to Oxfordshire on went down in the Thames Estuary on 5 January 1941), but no more has been heard about the elusive airplane since then. Sheridan has also long been beating the drums about the U.S. Navy's having a secret team of dolphins trained "to recognize and drown enemy divers," claiming in another Observer article back in 1998 that sixteen of the animals were destroyed with small explosive charges planted in harnesses on the undersides of their necks and detonated via radio signals "in order to prevent the dolphins and the Navy's technology from falling into the wrong hands" when they got loose off the coast of the French Mediterranean.
The U.S. Navy does employ trained dolphins for tasks such as detecting and marking mines, but even such intelligent creatures as dolphins aren't too good at the advanced tasks of distinguishing friend from foe or ordinary scuba divers from terrorists. Killer dolphins are likely to be found only in the realm of fiction and overactive imaginations.