The Herald reports that Dr Cullen may step down as Deputy Leader (and by default Deputy PM) in February.
The likely contest is Maharey vs Goff as his replacement as Deputy. Maharey would win easily.
However then after the 2007 budget, Cullen will also presumably step down as Finance Minister at which point Mallard would be odds on to become Finance Minister.
Then jumping ahead to 2008, with Clark stepping down (presuming a loss) you would have a fascinating leadership battle between Maharey as Deputy Leader and Mallard as former Finance Minister.