Some interesting information on the FDR pinger here:
Black Box: Locating Flight Recorder of Air France Flight 447 in Atlantic Ocean | MarineBuzz.com
It talks of the range being one mile, say 6000 feet, and that might be explained by the high frequency in use. In turn this suggests that th eonly way of detection will be by a towed array at a depth of 10000 feet. A two-mile length tow is a lot of tow and a 2 mile sweep width means a lot of steaming to cover the area.