The gossip has it that, on appointment as Middle East envoy for the Quartet, Mr Blair will stand down as an MP.
It is one of those quaint anachronisms of the House of Commons, however, that MPs are not actually permitted to resign. Mr Blair will therefore have to apply for one of two paid offices of the crown, which would disqualify him from membership of the Commons. In Mr Blair's case, he will need to become the crown steward of the Manor of Northstead. He does not actually get paid any money but it enables him to be excused boots in the Commons.
The other office (they take it in turns) is the steward and bailiff of the Chiltern Hundreds.
The august office of the steward of Northstead is currently held by Peter Mandelson. (Not a lot of people know that.)
Update: Apparently, Mr Blair has become the steward and bailiff of the Chiltern Hundreds rather than the Manor of Northstead.