I just discovered today (November 1, 2006, see the same link above for the reference) that the question of counting different Sudoku puzzles (the 9x9 case) was solved just this year, January 2006, by Ed Russell and Frazer Jarvis. They counted 5,472,730,538 "essentially different Sudoku grids". That's around 5.5 billion different possibilities of arranging the numbers. That means there are more than 5.5 billion Sudoku puzzles that can be constructed by hiding grids in many different ways.
Ton, naunahan na pala tayo kahit nagawa pa natin ito noong Summer. Hehehe.
