If Miers does withdraw or bombs at the hearings, my guess is that Bush will ask O'Connor to stay on for the remainder of the term, and start over with a new pick next summer.
Or at least next year sometime. The closer the nomination fight is to election day, the greater its potential impact on the election. One third of the senate seats are up for grabs and each side will be weighing the imapact of the fight on their respective bases against the risk of alienating the undecideds.
I hope and pray Bush picks someone who nomination will fire up the Republican base. In recent history, unlike the Democrats' base, we've shown the ability to deliver elections when we're fired up.