malaysiakini has this article 'Musical chairs' in Umno transition plan that shows the weakening position of the PM with respect to his staying on in office.
The most basic mistake he made was to display his weakness by agreeing to step down.
Even if he had a date like 2010 when he wanted to hand over, he should never have told that date to anyone; least of all to those who were waiting to pounce on any sign of weakness.
In politics, there are no gentlemen - everyone is waiting to step up and take your place.
Just see how the admittance of the preferred date of mid 2010 has been whittled down to the basic essentials - everyone now wants closure of his term. Those who want him out have used the Communist tactics of negotiating very successfully.
First they draw the line where you make the concession and then they extract more from that position. It has been very effective to change the date from mid 2010 to now perhaps March 2009.
Don't be surprised if he leaves even in late 2008 or early 2009. If he decides not to contest and Najib is given a large mandate from UMNO, that will surely be on the cards.