In his 2004 essay, Goedel and the End of Physics, Stephen Hawking conducted a U-turn. A past believer that the hunt for the Theory of Everything (or Grand Unified Theory - GUT) would be successful, perhaps even in the near future, Hawking decided that the same reasoning as Goedel applies to mathematics also suggests that such an all-encompassing theory is not possible.
Goedel's theorem states that no finite system of axioms can prove every result in arithmetic. Goedel draws on self-referring and self-contradictory statements to make his point. An example of such a statement is "This statement is false." Think about it. The statement he used was a mathematical one, and the proof involved other important points, but Hawking draws on this one specifically when he extrapolates from maths to physics.
If the system we seek to describe is the universe, then we have a problem - we can't step outside of the system to view it from on high. We, our apparatii and our equations are all part of it. So any physical theory seeking to describe and predict the universe is self-referencing in the way that Goedel's statement was. Perhaps, then, it shouldn't be surprising that the Standard Model of particle physics and the General Theory of Relativity (which explains gravity, the one thing NOT explained by the Standard Model) are incompatible.
Hawking's argument is not a proof, but rather an analogy. Even if Hawking is wrong in saying that the theory of everything is unreachable, his essay is useful reading for someone interested in the biggest questions about the universe.