Confinement of antihydrogen might help provide a future answer. http://arxiv.org/abs/1104.4982
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You're of course right. Experimentally established, known laws of physics - especially the equivalence principle - are enough to be certain that antimatter has the same gravitational properties - including universal attraction - as ordinary matter.
The justification of further experiments by "tests of antimatter's gravity" is partly based on ignorance and partly on deliberate deception to get funding.
Never seen any good reason to think that antimatter is affected any differently.
Light and anti neutrinos (from supernovas) seem to be affected by gravity the same as regular mass, and the theory of gravity (general relativity) doesn't distinguish mass and any other energy. If we beam a gamma photon down the gravitational well (from space to ground, for example), it gets blue shifted a little bit - it gains energy. If we use that gamma photon to produce matter antimatter pair and send the pair up the gravitational well, unless the antimatter is affected by gravity exactly the same as matter (and light), the conservation of energy will be violated. It makes absolutely no sense for antimatter to be affected by gravity differently than light, when the matter is affected by gravity the same as light. (note: no, there is no anti-light)
I guess it is the usual opportunistic fundraising, exploiting the ignorance. It is hard to raise funds for real research, realistically presented - it sounds obscure and layman has no idea what it is for - and it is easy to raise funds for something that layman thinks is a huge question with good likehood of something extremely awesome, like antigravity. When one is perfectly ignorant then the likehood is fifty-fifty.
See the paper http://arxiv.org/abs/1103.4937 by M. Villata for an argument for antimatter falling up. However, see D.J. Cross at http://arxiv.org/abs/1108.5117 for powerful rebuttal arguments, asserting that Villata misapplied the CPT theorem, resulting in an incorrect sign for the matter-antimatter gravitational interaction.
See http://lss.fnal.gov/archive/test-fn/0000/fermilab-fn-0822-cd-t.pdf for a general review of the previously existing experimental evidence from Etvos and other experiments such as accelerometers, Kaons, astrophysical neutrino's from SN1987A.