I know that a lot of Northern nobles were driven in to exile (or killed) during and after the Harrying of the North, but I've always wondered what happened to the less hostile nobility. Did they just drop down a class and become part of the lower orders? If they did retain their noble status, are there any medieval noble houses which were founded by an Anglo-Saxon?
One theory I've heard espoused was that the Anglo-Saxon warrior ethos of the Huscarls (the core bodyguard/army of the King) was so strong that many fought to the death at Hastings, thereby depriving England of much of its nobility in the aftermath. Coupled with Williams further campaigns, systematic handing of land over to continental nobility and intermarrying his followers with existing landed families, this could serve to answer your question.
Another example of Huscarls/household troops fighting to the death is the famous Battle of Maldon, wherein the local Earl's men fought to the last man even after the death of their leader.
The Feudal Kingdom of England 1042-1216 By Frank Barlow does a great job of analyzing the times before and after the conquest, as well as some of the details involving the transition itself.
The Battle of Maldon by E.V. Gordon should give you an overview of the epic poem written about the battle as well as insights into the actual historical event.