Kotaku has an interesting article about the accuracy of Red Dead Redemption, but this video sums most of the game up. Rock Star San Diego is the studio responsible for this. They are also the ones responsible for the Midnight Club and Smugglers Run series and of course Red Dead Revolver. I never played Revolver, but looking at the world in Redemption it appears to be huge. They seem to have taken some inspiration from their coworkers work on GTA:San Andreas and made a large living world.
This game definitely deserves a closer look.