The U.S. Supreme Court denied Michigan’s request to take immediate action to stop Asian carp from reaching the Great Lakes. In a one-line order (go to page 3), the Supreme Court denied the State of Michigan’s motion for a preliminary injunction, siding with the State of Illinois, the federal government (essentially the U.S. Army Corps of Engineers), and the Metropolitan Water Reclamation District of Greater Chicago. "Motion of Michigan for preliminary injunction denied." Wisconsin v. Illinois, 558 U.S. 1145, 130 S. Ct. 1166, 175 L. Ed. 2d 970 (2010). While the Supreme Court did not issue an opinion explaining its decision, it’s fair to assume that the Court was persuaded by the brief filed by the Solicitor General on behalf of the United States. The federal government sided with Illinois and asserted that the federal government is already doing everything possible to protect the Great Lakes from Asian carp (a doubtful assertion at best).
Update: The renewed motion of Michigan for preliminary injunction was also denied. Wisconsin v. Illinois, 559 U.S. 1003, 130 S. Ct. 1934, 176 L. Ed. 2d 359 (2010).
Update: The Supreme Court has also rejected Michigan’s petition for a new decree under the old Chicago diversion case, Wisconsin v. Illinois, and Michigan’s alternative request to open a new case. "Motion of Michigan to reopen and for a supplemental decree denied. The alternative motion for leave to file a bill of complaint denied." Wisconsin v. Illinois, 559 U.S. 1091, 130 S. Ct. 2397, 176 L. Ed. 2d 765 (2010).