In a forum thread at HBO discussing recent claims that Halo saved game files had been hacked to gain access to infinite ammo and other things, Mat MSN Noguchi had the following to say:

All things considered, I doubt anyone has managed to hack the save file of any game yet.

Besides, there are much easier ways to hack Halo :)

Louis Wu replied that it was mean to say that without providing details, but I think it's safe to say that if there are other easter eggs or exploits left to find, Bungie will probably leave the fans to find them on their own.