Well, it appears that the mystery of the Halo 3 Hayabusa helmet has already been solved with persistent fanboys over at Halo3Planet.com figuring out how to obtain the "I Would Have Been Your Daddy" skull.
To refresh your memory, the IWHBYD skull was the only skull that needed to be found, because the Hayabusa helmet (as part of Halo 3's armor permutations) can only be unlocked when all thirteen Halo skulls have been found. Thus, IWHBYD is uber important. Anyhoo, like we said, the skull has been found and is quite the tricky little devil to get. Full instructions can be read over at Halo3Planet.com, but in brief you'll have to make your way to "The Covenant" level and jump through seven Halo holograms in order to play the Halo theme song music. Crazy huh? We're sure Bungie thought they could keep the wraps on the IWHBYD skull for a while, but never underestimate the fanaticism that is the Halo community. Now go, go get your thirteen skulls and unlock the Hayabusa helmet ... your l33t status is in jeopardy!
[Thanks, to everyone who sent this in]