Following up on this guide to the Courts, here's the Family Research Council's take:
Today the Los Angeles Times reported that Supreme Court nominee John Roberts did pro bono work on the 1996 Supreme Court case that resulted in the striking down of a Colorado state constitutional amendment that prevented local government from offering protected minority status or preferences based on homosexual or bisexual orientation or conduct. Judge Roberts did the uncompensated legal work on the case, Romer v. Evans, while he was an attorney for the DC law firm Hogan and Hartson. After further investigation we were told that Roberts' role was apparently limited to providing a few hours of participation in a moot court procedure, as he routinely did for all the firm's pro bono clients. More on this as we learn more about this report.
Sorry, no link, this is from an email update. Looks like the facts are still coming in on this one. Did he moot court or did he advise? There would be feedback and advice in a moot court proceeding, but that's different than picking the client and actively helping them prepare their arguments.