In the US, who cares, saying you lost both engines and were landing in the Hudson doesn't require a mayday call. Even answering ATC if they can't help you is a waste of time in an emergency. Notice on the transcripts ATC did not ask for those annoying reports on souls on board, fuel, etc, but left them alone to deal with the ditching, probably required if you declared an emergency, or Mayday for the brits.
I've read in the forums on liveatc.net that ATC can in effect "declare an emergency" for you, if they decide conditions warrant, in terms of their internal procedures. There are a number of emergencies recorded there where ATC is asking for the souls on board and fuel - which is important information for the rescuers - even when no "mayday" or "declaring an emergency" call was made.
All in all, relatively unimportant here... as are the 911 calls that were played in the media. It's not like all those ferryboat crews were waiting for a call from ATC to go rescue anyone...