He is dammed if he does and dammed if he doesn't. He did not stand up and make an announcement - it came out in an informal interview. When Mrs Thatcher said she would "go on and on", she was condemned. Although a Prime Minister is not a President, the US constitutionally stops the leader doing more than 2 four year terms.
The press have overblown the issue in my view. Anyway he may well not be the PM after May, anyway.
If, by any chance, Labour lose, I wonder if David will replace Ed (after getting a safe seat)?