The best Sudoku puzzles are written by humans. Computer Sudoku generators still have to be relatively sophisticated in order to achieve a target level of playability. The easiest Sudoku puzzles are rediculously simple, and the hardest literally require branch investigation down possibly multiple branches for a number of levels before the person solving the puzzle can determine which branch is going to play out to the end. In fact, some can be rendered unsolvable without resorting to guessing. Puzzles generated at random are going to sometimes be unsolvable unless a person gets out a pen and paper and starts making assumptions (guesses) and pursuing branches until they can be proven to not be a viable solution. In fact, in the worst cases, it may be impossble to take a set of 'givens' to the point of eliminating all but one solution.

Part of the problem is in deciding what shall be the 'givens'. The givens have to be chosen in such a way as to make a puzzle solvable without pure guessing. In other words, they should set up a relationship that can only lead to one unique solution.

The Wikipedia contributors have done a better job than I'm doing of explaining my point, and their eplanation can be read at http://en.wikipedia.org/wiki/Sudoku. In particular, the discussion of construction should be of interest. Of course you're a bright individual, and I wouldn't put it past you to develop a very well thought-out implementation. I just wanted to contribute a few caviets that might help you toward creating a more refined gadget.

Good luck! I'd love to see the outcome.