This is arguably a corollary to the Heisenberg Uncertainty Principle.
We can't know the universal speed limit, as anything faster than c (the speed of light) would be unobservable to us, but that doesn't necessitate that it can't exist.
This idea flies in the face of a paramount premise in Einstein's Theory of Special Relativity, wherein he asserts that nothing can travel faster than the speed of light, as light isn't so much a particle or wave, but a phenomenon indicative of how the universe works. This tenet was since debunked as the speed of light did, in fact, turn out to be variable when it was shone through different materials (gasses, glass, and the nucleus of certain exotic atoms), and when affected by gravity as in the case of Gravitational Lensing, which was initially thought to be proof of the Theory of General Relativity.
Update 10/25/2010
The above phrasing is not strictly correct, as we could still observe something going faster than the speed of light, with some minor exceptions. The idea came about while relating the speed of sound from the perception of a blind being to light.
Let us pretend that we are blind cave fish, and have never experienced sight or anything having to do with electromagnetic radiation (light). The speed limit of our perception would be the speed in which sound travels through whatever medium we find ourselves in. All the while light would be travelling much faster than our arbitrarily assigned universal speed limit. Lorentz transformations would involve the speed of sound instead of the speed of light, and relativity would work in just the same way as it does now. However, we would be wrong in our assumption that nothing travels faster than the speed of sound (in our case: light), which are mediated upon our lack of visual senses (in this hypothetical situation). Therefore, just because our senses are limited to a certain speed does not necessitate that nothing can move or interact faster than this speed.