I have a tough time buying this, and here's why:
1) The president and his advisers KNOW that the judiciary is a hot button issue that could cost them their electoral majority. If they drop the ball on this (because of a supposed fear of fighting with libs), they tarnish everything that they've done, and Bush will be judged to have broken his word.
2)The president, to his credit, has NEVER been someone to back away from a fight. Also, he personally knows the stakes of an activist judiciary (see Bush v. Gore) and is committed to reforming it. I think that he believes that Miers is a predictable vote that he can count on for years, and who can be confirmed with a minimum amount of effort.