First off, Hudson stated that Mass Effect will fit on one disc, but "just barely." Then he went on to talk about how players will be able to tweak the main character, Commander Shepherd (A.K.A. Jack Bauer in Space) to their liking -- gender, facial features, class, starting skills, and more will be determined not by the developers, but by the player.
Hudson also commented that while he and the development team "don't mind supporting" custom soundtracks, the game's soundtrack is a "huge original score," that will add to the ambiance.
Does barely fitting on a disc as of the moment mean it's possible that Mass Effect could be one of the first multi-disc games for the Xbox 360?