How Much Do Supreme Court Opinions Borrow from the Briefs?:
Over at the Glom, David Zaring flags an interesting new paper about Supreme Court opinionwriting: Pamela C. Corley, The Supreme Court and Opinion Content, Political Research Quarterly, Vol. 61, No. 3, 468-478 (2008). Here's the abstract:
Do parties' briefs influence the content of Supreme Court opinions? The author contends that the parties, through the briefs submitted on the merits, have the ability to influence the content of opinions and, consequently, have the ability to influence the law. Utilizing plagiarism software, the author compares the parties' briefs with the majority opinion of the Court. The results indicate that there is a connection between the language of the parties' briefs and the language of the opinions, which means that parties have the potential to influence the law.
  The paper unfortunately is gated right now, but those coming through academic institutions with access rights can get the paper here. Those who can't download the paper can get David's summary:
As you might imagine for a very unbusy Court overloaded with law clerks, time, and amici, the Court didn't borrow too much from the party briefs when it writes its decision, at least for the opinions issued in the 2002, 2003, and 2004 terms. The average overlap between opinion and winning party brief then was roughly 10%. I bet the percentages are higher in the appellate courts, and especially in the district courts.

Sometimes, however, the Court found winning briefs to be, shall we say, highly persuasive. In one case, Justice O'Connor used 41% of a respondents' brief in her opinion, and in another, she used 33% of an appellant's brief. Rehnquist and O'Connor were the justices most likely to borrow from the briefs (they comprised 14% WJR/11.5% SDO of the content of the justices' majority opinions authored during those three years, depending on respondent/appellant), Souter the least (7% either way).

Corley found that the justices are more likely to borrow from high quality briefs (proxied by a DOJ or DC return address), from ideologically compatible briefs (conservatives were more likely to use briefs advocating the conservative position), and from briefs in low-profile cases, or at least low-profile enough not to appear on the front page of the New York Times the day after the decision was handed down (which might just mean "statutory cases").