[math-fun] sudoku questions

The sudoku puzzles that the L.A. times has started carrying have me wondering how the puzzles are created and validated, and especially how it would be done without resorting to brute force computer techniques. I'm guessing the puzzles start with the solution, then remove squares until some desired degree of difficulty to reconstruct the solution is achieved. The Times puzzles are graded "easy" "moderate" and "diabolical". 1) Are all "solved puzzles" the same intrinsic complexity, or are some intrinsicly easier than others. 2) How do they validate that the solution for a puzzle is unique, and how do they grade the level of difficulty.