A novelty. I think the significance of that piece was one of the worse art-house declarations of the last century...it turned away a lot of potential art-appreciators in droves.
So he made a statement...he could have done that without all the bru-ha-ha. The repercussions remain: I can't count how many times I've talked art music with people and they've brought up 4'33".
Perhaps Cage's real statement with all this is how easy it is to use "art" instead of "ah!...clever."