Fergie is one of the few "unsackable" managers along with the likes of Dario Grady, Bobby Robson and possibly Wenger who practically has a job for life. The only way I can see him being pushed (and it would most likely be upstairs rather than out the door) is by a combination of off the field incidents (dressing room bust-ups, letting too many cats out the bag to the sunday papers etc) and failure to secure silverware. So by that reckoning, I guess he is at a big a risk as he has been for over 10 years. However bear in mind he is only in the first year of a 3 year contract, and I'm sure its quite a juicy one too which would cost a few quid to pay up.
IF (and it's a big if) he were to leave, I think they would have to look overseas for a replacement of high enough calibre. Robson and Wenger have the talent, but Robson would be viewed as too old and unlilely to leave Newcastle, while Wenger wouldn't be so foolish as to quit on all the hard work he has put in at arsenal. Most of the up-and-coming young managers mentioned above like Strachan, Moyes and Curbishley just haven't had enough success yet. Personally I don't think people like Moyes could handle the big stars at Man Utd anyway. The only englishman I can see taking the job would be Steve Mclaren going back to his roots. Expand that to British managers and George Graham and Martin O'Neill enter the frame.