Sorry, Flynn, but it's not NIN-JA GAY-DUN, it's NIN-JA GUY-DEN. As in N'Gai Croal. I officially win our debate. Team Ninja's master ninja Tomonobu Itagaki—don't worry he helps with the pronunciation of that, too—provides the sound-off in the latest MTV Multiplayer pronunciation guide, even throwing in the proper way to say "Helena", something I wasn't even aware was an issue. Sorry, Canada and the UK, you know the deal with this MTV video nonsense. Ask a friend in the U.S. to give you the gory details.
How To Say 'Ninja Gaiden' The Right Way, From Itagaki-San Himself [MTV Multiplayer]