The specific question is whether the reservation once afforded the Creek Nation in what is now eastern Oklahoma remains a reservation for purposes of the Major Crimes Act. If it remains a reservation, then the state of Oklahoma was powerless to try Patrick Murphy for a murder he committed on that land in 1999, because the Major Crimes Act requires federal prosecution of certain major crimes committed on Indian reservations. If the land is no longer a reservation, then Oklahoma retains the authority to prosecute crimes in that territory.
Read all of Ronald Mann at SCOTUSblog on a very interesting tribal reservation case to be heard by the U.S. Supreme Court.
History has a way of catching up with things. Oh, and here's a line of particular interest:
A ruling in favor of Murphy would suggest, among other things, that all major prosecutions in eastern Oklahoma since 1907 were invalid as a violation of the Major Crimes Act.