I did look in th rules compendium on his. It was sketchy at best but i think the possibility of denying a dex mod to the save was mentioned. On another note, I seem to remember in older editions like 1st and 2nd that a breath weapon save was explained as getting out of the way or hiding behind something, so to play devil's advocate in defense of that veiwpoint, is it possible to deny teh save since your prone and there is no way you could move out of the way or hide behind something? I'm just looking at all the angles of thought here.