SudokuU
This Sudoku app will run on the iPhone and iPad. A unique feature is the highlighter that allows the user to highlight specific numbers in the play area for easier identification. This app also allows the exportation of the playing surface to an email for later printing. You can export the puzzle as well as its solution.
This app also includes what I call an X-beam feature that makes the diagonals of the puzzle comply with the rules of Sudoku.