Well, the point of science is that we don't know until we observe it. So it shouldn't be that astounding since we've had a ridiculously hard time getting large amounts of neutral antimatter (everything before was charged, so EM affects overwhelmed gravitational). The consensus seems to be that antimatter will behave 'normally'. but we haven't seen it yet. Also, it turns out that certain frameworks where anti-matter is repelled by matter gravity doesn't violate anything else that hasn't already been violated (for example, CP and CPT symmetry have already be observed to be broken). And thus the uncertainty.
Hold on, where are you getting the CPT-symmetry violation from? This paper http://arxiv.org/abs/0801.0287v4 has a collection of experimental results hinting at that but the conclusion does not as yet seem to be very strong.
I've never heard anyone argue that it would react any differently to gravity than normal matter. After all, the only difference is the electrical charge of the particles. They have the same mass.
Of course, regular old hydrogen "falls up", being lighter than air. :) I guess if gravity is truly reversed for antihydrogen, it would fall up even faster. But I highly doubt it's actually gravitationally repulsive.
Or perhaps must be affected by antigravity the way matter is by gravity. Assuming antigravity exists. Whether that's correct at all I don't know, but that's the logic being used.
So if it "reacts the opposite" to gravity, what does that mean?
I would see no reason for Newton's law to not hold true around our scale, so repelling means F=Gm1m2/r^2 which only makes sense if one of the masses is actually negative. Just the thought of it looks like fun.