I don't think there's a hidden meaning behind all this. If there is, the 6 would probably stand for the 600D, because a 6D wouldn't make much sense in the current lineup - the 5D2 already is the "baby FF camera", with most of it's specs bested by the "old" 50D (except for the sensor and some software features like video).
If this was Apple, there sure would be a hidden meaning behind it, but I can't remember seeing Canon doing stuff like that.