Besides Holy Idols, which casts infravision, do you think holding Items actually have an affect on Thac0 or other aspects of the game?

Some affect thac0, but most do not. Unfortunately since you can't see your character's thac0, there is no way to know for sure. Also, some increase your character's "luck" which can be deteremined with the fortune spell. Again, there's no significant evidence that this actually changes anything.