According to Spong.com, Super Paper Mario on Wii will be released September 14th in Europe.
In case you've never experienced dead-tree Mario, here's the rough idea according to Nintendo, "As with the previous two games in the Paper Mario series, all characters are as flat as a sheet of paper. However, our hero Mario can manipulate the environment around him to his benefit, opening up new paths and possibilities with the ability to 'flip' from 2D to 3D at will. This ability comes in handy throughout the game and is crucial to the progress of the player in his rescue mission."