As Matt observes, Justice John Paul Stevens gives no signs that he’s thinking of retiring. For what it’s worth (and I don’t mean to suggest that I have any insights into the factors that may affect Stevens’s decision to retire), I’ll note that Justice Stevens is not far from setting records for both longest-serving justice and oldest justice.
According to Stevens’s Wikipedia bio:
Stevens is the eighth longest-serving Justice in the history of the Court.* If he continues on the Court beyond July 15, 2012, he will surpass the current record of 36 years and 209 days held by former Justice William O. Douglas.
Stevens is the second-oldest Justice in the history of the Court. If he continues on the Court beyond February 24, 2011, he will surpass the current record of 90 years and 310 days old held by former Justice Oliver Wendell Holmes, Jr.
* Update: This separate Wikipedia page has Stevens as the ninth longest-serving justice—and its day count for him appears to be current.