John Reid has announced that he will step down as Home Secretary when Tony Blair goes. He says he wants to "recharge his batteries" on the backbenches. He is lucky to be able to choose (to a certain degree - although it's a 'jumped before he was pushed by Gordie' situation) when he leaves the Home Office - most Home Secretaries don't.
I suspect he could not work with Gordon Brown. It has already been speculated that Reid was the minister who told Nick Robinson that “Brown would make a ****ing dreadful Prime Minister”.