When experimenters studied the quark-gluon plasma, a large chunk of excited nuclear matter (of colliding golden nuclei), they thought it would have to behave as a gas or plasma: the temperature was so high and the stuff was so hot! Instead, what they saw was that the material actually behaved as a liquid - a seemingly cold type of matter.
And it wasn't an ordinary liquid: it was actually a liquid with a very low viscosity. In fact, it had the lowest viscosity that a liquid of the same entropy density could have: a perfect liquid! Despite a trillion of degrees Celsius, it behaved like a pretty cold liquid. Meanwhile, the string theorists already knew why this thing would be observed.
Surprisingly, an ultracold "gas" - Lithium-6 that was cooled just to dozens of nanokelvins in 2002, which is a temperature 19 orders of magnitude cooler than the previous, nuclear stuff, behaved in the very same way. A perfect liquid, too.
The minimum ratio of viscosity and the entropy density can be translated in another way: it is actually the maximum ratio of the entropy density to viscosity. For a fixed viscosity (and volume), which physical system has the highest entropy density (and therefore the net entropy)? Well, in the gravitational context we know the answer. Black holes maximize the entropy. They're the ultimate bound state of matter into which the matter collapses into, and by the second law of thermodynamics, they must maximize the entropy among all such bound systems.
It turns out that string theory guarantees that these fully thermalized systems necessarily try to maximize the entropy-to-viscosity ratio. It's because these systems have an equivalent, i.e. dual description on the boundary of an auxilliary AdS space with one additional dimension: the dual description of this matter is nothing else than black holes. It has to be because it is a theory of quantum gravity where black holes inevitably arise under mild assumptions.
The picture correctly describes not only all the surprising qualitative properties of this type of matter - including the ability of the plasma to "catch" even heavy quarks - but also many quantitative features - such as the viscosity-to-entropy_density ratio itself. These experiments prove that the gravitational force is unified with other types of forces and matter in ways that people couldn't have even imagined - and in ways that can only be explained by string theory.
It used to be thought that the gap between the typical energy where quantum gravity takes over - the huge Planck energy - and the characteristic energy scales of other types of forces will remain huge forever. Even reasonable people thought that it would mean that no conceivable unification could ever have observable consequences.
However, the observation of the strongly-coupled systems above show a very different picture. Those systems are equivalent to a warped geometry where a stringy theory of quantum gravity lives. Because the length scales (and, inversely, energy scales) in the ordinary four-dimensional spacetime depend on the position in the fifth dimension (according to the warp factor), the overall scaling of the distances in the 4D space kind of depends on the type of physics one considers. Each type of physical phenomena has its typical scale which gets translated to different positions in the fifth dimension.
The proper distances in the five-dimensional bulk can therefore differ by large factors from the distances as seen by the projection to our four-dimensional spacetime. Well, all these things mean that Newton's constant derived for the dual gravity of many of the systems above may be very different from our four-dimensional Newton's constant: but it can still describe the same gravity because this Newton's constant may be obtained from a higher-dimensional one by a different truncation.
This dual bulk AdS gravity is just one way how to connect those seemingly non-gravitational systems with physics of quantum gravity. Who could have thought that the unification of ordinary forces with gravity will force us to solve the strong coupling limit of mundane pieces of matter - such as the quark-gluon plasma or Lithium 6? However, string theory implies that many more links, dualities, and transitions exist between the gravitational force and other forces. All of them are true and all of them are pretty much inevitable as long as one believes the experiments above as well as straightforward mathematical derivations.
The basic strategy how to unify gravity with other forces used to seem "obvious" in the past. But Nature has prepared many surprises to us. Even extremely non-gravitational - and, indeed, mundane low-energy - forces and phenomena are equivalent to a quantum gravitational system in string theory. Some forces get unified with others if their backgrounds are non-trivial (e.g. curved); others do so if their coupling constant or the density of particles is very high (or strong).
AdS gravity gets unified with the forces governing the ultracold Lithium-6 in a tighter way than anyone has imagined as recently as 15 years ago: they're completely equivalent. The relevant space is asymptotically AdS, which controls the high-energy phenomena, while the geometric arrangement near the "center" of this space carries the same information as the detailed low-energy properties of the quark-gluon plasma or the Lithium-6 gas.
The existence of the normal, four-dimensional gravity implies that the new fifth coordinate has to be truncated, so that energy scales exceeding the ordinary four-dimensional Planck scale are eliminated. Such a truncation has to follow some consistency rules - including a "continuity" with the bulk physics - and these rules are determined by string theory. This point is meant to emphasize that the ordinary four-dimensional gravity is inevitably connected to the "auxiliary" gravity used to describe the "perfect liquid" systems.
As we have mentioned, besides this unification, there are other insights that unify the forces in other ways. For example, a graviton can be continuously transformed into a photon through the Kaluza-Klein mechanism (compactification). Such a photon may be geometrized as a deformation of a singularity, an excitation on a D-brane, and so on. The whole world is thus represented as a grand quantum generalization of Riemannian geometry we are familiar with from general relativity, something that Einstein always wanted. It is filled with lots of links between objects and concepts that were thought to be independent and different in the past.
Einstein couldn't have found the right picture that unifies all the forces and matter in this way because he dismissed quantum theory and all the majestic equivalences, dualities, and transitions discovered by string theory only work if quantum mechanics is taken completely seriously: they finely depend on it. But it is clear that if Einstein were ultimately able to accept that quantum mechanics was inevitable, he would have known that string theory has realized all his dreams and many more.
All these things are amazing and I simply cannot respect anyone as a theoretical physicist if he or she is alive and if he or she hasn't understood the inevitability of all these conclusions about unification of forces even in 2009, decades after string theory has become pretty much established. And indeed, when suggesting the lack of intellectual respect, I am not talking just about the anonymous brainwashed human waste hiding behind another piece of the same waste that gathers on various crackpot forums such as Peter Woit's forum.