According to the latest news blurbs about the rape allegations against Duke lacrosse players, the prosecutor requested that a witness not reveal that the DNA tests showed no DNA from the three suspects. I'm not a big fan of hurling ethics accusations based solely on one side's version of the facts as reported in the news, but according to the news stories, this is not simply an assertion by the defense attorneys; this was self-implicating testimony from the witness himself while being cross-examined. Nifong ought to address this issue immediately. If the witness's testimony has been accurately described, Nifong violated the standards in Model Rule 3.8, and quite possibly MR 3.4 and 3.6 as well. (Instapundit has more, including news of a press release from Prof. Banzhaf of George Washington.)
UPDATE: If Nifong asked a witness to conceal exculpatory proof, there could be a lot more on the line than just an ethics violation. Civil and even criminal liability could become issues. For a recent article on prosecutorial misconduct and its institutional causes, see Peter Joy's article here.