Stevens has hired only one law clerk. Most justices hire four, and Stevens has almost always hired all of his clerks by June or July.

The AP:

In response to a question from The Associated Press, Stevens confirmed through a court spokeswoman Tuesday that he has hired only one clerk for the term that begins in October 2010. He is among several justices who typically have hired all four clerks for the following year by now. Information about this advance hiring is not released by the court but is regularly published by some legal blogs.

Stevens did not say whether he plans to hire his full allotment of clerks or whether he will leave the court at the conclusion of the term that begins next month. Retired justices are allowed to hire one clerk.

As with the Souter replacement, Obama wouldn't really have a chance to change the ideological makeup of the court, since Stevens is a reliably liberal vote. But he would potentially be able to replace the court's oldest justice with the court's youngest.