Bungie is holding a compo that will get the lucky winner's emblem design into Halo 3.
Bungie's community mouth-piece Frankie's says, "David Candland, our UI artiste extraordinaire, asked, 'Wouldn't it be fun to have a Bnet reader design an emblem and include it in the shipping game?' Well wouldn't it? Of course it would. So we're going to give one lucky Bnet reader the chance to design an emblem to be emblazoned forever in Halo 3."
You can read the rules here but your emblem must be original. Don't copy anything as entries that look like other logos or corporate images will be ejected (same goes for National flags), says Bungie.