Sinnoh is the region in which Pokémon Diamond and Pearl are set, along with Pokémon Platinum. It is a widely varied area, with a large chain of mountains dividing the region in half.
In Super Smash Bros. Brawl
The Pokémon stage, Spear Pillar, is located in Sinnoh. Spear Pillar is supposedly the area where the Sinnoh region (if not the entire Pokémon Universe) was created and is thus very important in the game. The protagonist of Diamond or Pearl battles the leader of the evil Team Galactic, Cyrus, here and captures or defeats either Dialga or Palkia. Much later in the game, it is possible through the use a special item to battle the strongest and rarest Pokémon of all, Arceus. Arceus does not appear in Brawl (an enormous star at the top of the background at Spear Pillar resembles Arceus though), as it hadn't been officially announced, though Dialga and Palkia both appear as background characters on Spear Pillar. Cresselia is also seen appearing and using Psycho Cut.
The Sinnoh region may also be home to the stage Pokémon Stadium 2 as 2007 is the release year of Pokémon Diamond and Pearl; however, this stage could be practically anywhere, considering that Pokémon battling is common to most of the Pokémon world.