I was playing with sudoku's a while back and made the following spreadsheet.

http://spreadsheets.google.com/ccc?k...HIh_g1XA&pli=1
There are 3 grids.. the upper left grid starts with the original puzzle and as you discover answers you fill it in.

Now.. if the answer in the left grid is found.. each cell in the upper right grid will have that value. If the answer in the left grid is not found.. the cell will contain every number from its corresponding cell in the left grid that is horizontal, vertical, and in the same 9 block cell from it. It does this just by treating them as text and concatenating them together.

Finally.. for each cell in the lower right grid.. check its corresponding cell in the upper right grid. If its less than 10.. its the solution. If its greater than 10 then build a string out of every number not used. These are the possible numbers that can be in that cell.

The upper right grid in a program can probably be dropped. Its pretty redundant but the formula in this form would have exploded had I tried to do it in one pass. Now.. i've included a sudoku unless someone has changed it and you'll notice when you put one in that the lower right will end up having cells which break down to one number. When this happens you take that number and put it in its corresponding cell in the upper left. This will then have a cascade effect and drop out more numbers.