According to a Dengeki Bunko Magazine, Shakugan no Shana has been confirmed to get its third season in Fall 2011. The title says 'Shakugan no Shana III Final' which indicates that they will most likely conclude the story this time. However, the original light novels haven't been finished yet, so it's very probable that the creators will come up with their own way of ending the series. The previous season didn't follow the rail of light novels anyway.