While unobtanium's exact nature and role is never defined within the film itself, it's observed properties seem to indicate that it is either superconductive (meaning that the mechanism by which it "floats" would be magnetic in nature), or anti-gravitic in nature. Before you start hollering about "anti-gravity=fantasy" I would remind you that we now know of and are starting to understand the nature of how mass and gravity is "attached" to matter, via the Higgs boson. Therefore in a scientific context, anti-gravity could be explained as a variance in the way that Higgs bosons attach to the unobtanium, altering their mass and hence the effect of gravity on them. Either they attach in a way that is oppositional to that of boson attachment in normal matter, or possibly that unobtanium's bosons are "negative" bosons or "anti-bosons". And before you bring up M/AM and "boom", since bosons do not appear to have any actual mass themselves (as I understand the reading), no mass=no energy release. An "anti-boson" would simply cancel out a boson, rendering the unobtanium either mass-less or at a negative mass (assuming a surplus of anti-bosons. So, again, cause and effect completely within the bounds of conventional science or a reasonable extrapolation thereof, which is the very definition of science fiction as opposed to mysticism-based fantasy.