Is a die implemented in a physics engine truly random? So, a fair die throw is really random, not pseudo. So, would a RNG implemented as the result of a die throw in a physics engine (say, Newton, Havok, Nvidia's PhysX) be regarded as both cryptographically secure and really random (Not a PRNG, but a real RNG)?
No, that would not be a true RNG, because these physics engines would just repeat the exact same calculation and thus repeat the whole sequence of random numbers - like a PRNG. The starting conditions are the seed of this PRNG.

Dice are truly random in the real world. Well, are they? If we ignore quantum effects, we could measure all relevant values of the environment and then calculate the next value of the die - at least in theory. In practice the calculations are much too difficult, and the quantum effects destroy even the theoretical possibility to calculate the result.

The physics engines mostly calculate only the important parts of the calculation: the collision with other objects and the gravity. But in the real world, we got air movement (wind, breathing of a human), fluctuation in the temperature, very small movement of the earth, difference in the gravitational field of Earth, or even the vibrations of the heart, wear and tear of the dice or just contamination in the material. 

Even if the engine could calculate all this in an acceptable time period, it would have to introduce a RNG itself to simulate the quantum effects. If the engine does this (like mentioned by mikeazo in a comment), then why use the engine and not the RNG directly? If the output of the engine is bigger than the input from the intern RNG, it can't be a true random number generator anymore. It would have to stretch the random numbers. Maybe it's still secure, but using AES in CTR mode would do the same with surely much better properties.

Fun fact: There's the Dice-o-matic which again and again rolls many dice to determine random numbers. It's used to get random numbers for an online game platform. Well, that could be the closest you will come to get true random numbers from a machine with dice.