On January 18, the 10th circuit heard arguments in Constitution Party of Kansas v Biggs, 11-3152. The issue is whether Kansas must let voters register into parties that are not ballot-qualified. The Kansas Secretary of State himself, Kris Kobach, is an attorney, so he went to Denver to represent himself. See this story.
In 1984, the 10th circuit ruled in a Colorado case that states must let voters register into unqualified parties, if those unqualified parties have been politically active in the state. That case was called Baer v Meyer. Since then, other courts have agreed, including the 2nd circuit in a New York case, the New Jersey state courts, and U.S. District Courts in Iowa and Oklahoma.
However, the argument seemed to go badly for the Constitution Party. Two of the three judges seemed to believe that Baer was wrongly decided, and they may overrule it.