Simon Case is apparently going to be named as the UK's top civil servant. Although an official announcement will not be made until tomorrow the BBC is reporting he is to become the next Cabinet Secretary
The role is described as:
The cabinet secretary is the most senior civil servant in the United Kingdom, and is based in the Cabinet Office. The person in this role acts as the senior policy adviser to the Prime Minister and Cabinet and as the secretary to the Cabinet, is responsible to all ministers for the efficient running of government.
If true (doubt the BBC would report it if they weren't pretty sure it was) then it shows the calibre of staff the Cambridges have had but is also a potentially a risky move for Case, the current government seem no strangers to throwing their senior civil servants under the bus - the former Cabinet Secretary was asked to step downy the PM.
I guess William will be after a new Private Sec now.