The Democrats and Rebublicans in the Senate and Bush have been talking about nominees for a long time. The Democrats (at least the ones in red or red-leaning states) are dying for political cover. They don't want to filibuster. They know the kind of person Bush wants, but they need someone they can vote for without getting their brains beaten out by their base and big donors. Why would Reid come out and immediately express a measure of support? Because he very well might be able to make a quasi-defendable case to his own supporters for confirming her. And if a couple of years down the road she turns out to be a disaster for the left, well he can claim he was fooled (and that's an eternity away, anyway, in political time). And that's politics.
He's already backtracking, just like he did with Roberts.
But I doubt a lot of the folks looking to Reids initial support of Miers as a strike against her will turn around and view Reid's backtracking as a positive for her. They are interested less in getting a conservative nominee on SCOTUS than in provoking a fight - even though we got hamstrung by 7 RINOS during the last fight over the nuclear option. Some folks just don't learn. Bush apparently has and adapted his tactics.