Howdy,
The authors of the cited thread are crackpots, but gravity is the least understood of the 4 fundamental forces. Although Newton's inverse square law is very accurate at low velocities and Einstein's general theory of relativity extends this to high velocities, there are a couple of items that are unsolved problems in physics:
(1) quantum gravity: Einstein's general theory of relativity is the most accurate explanation of gravity that we have to date but it is not a quantum theory. The other 3 forces (electromagnetism, the strong nuclear force, and the weak nuclear force) have been quantized and have corresponding particles that carry the force. The other 3 forces are much stronger than gravity at very small distances and so the corresponding force particles are more easily observed. Furthermore, the 3 quantized forces have been integrated into 1 theory (the standard model of physics) but gravity has not been included. That is one of the promises of string theory is to integrate all 4 forces into 1 theory.
(2) the rotation rate of galaxies: If you observe the rotation rate of your run of the mill spiral galaxy, you will find that the arms do not rotate according to Newtonion gravitational theory. In fact, this is a big problem in astronomics right now. The "solution" to the rotation problem is to assume either one of 2 things: (1) there is extra gravitational matter that is not seen (i.e. dark matter) and (2) the inverse square law of gravity is slightly off. The dark matter "solution" assumes that there is a great deal of matter in a galaxy that has mass but doesn't participate in the strong nuclear force. The inverse square law "solution" makes very small corrections to the inverse square law of gravity to yield the observed rotation rates. Here's the Wikipedia link to this theory:
http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#The_change
So there it is. Take it with a grain of salt as I am not a physicist, just a run of the mill electrical engineer that spends too much time on Wikipedia.
Sholin