there are a few possible scenarios:
The first being that, of course, Santa is able to achieve a velocity near or equal to the speed of light. At the speed of light, naturally, the very essence of time stops and stands still. Thus, relative to Santa, the world has stopped moving and remains stationary: he is able to hit every house on the planet because we are immobile, and the planet is immobile. Relative to an outside observer, you can never see him because he moves much too quickly.This leads us to The Law of Improbability and its association to Absolute Position.
Schroedinger's Theory helps to explain to a degree the association between these two: in that, because atoms move at such great velocities, there is an infinite amount of possible places that an atom can occupy at any given timeframe(a second, a millisecond, a billionth of a second,etc.)and choosing the absolute position of that atom is highly improbable. The atom's position can only be attained by observation, and until then, it is everywhere at all instances of time.
However, physics also states that no body of mass can move at the velocity of light except the mass of light its self. And, considering that Santa is often portrayed as an overweight fellow, this means that the reindeer must exert much more force to try and achieve such a velocity, particularly since he is still within the gravitational pull of the planet.
The theory I suggest is that Santa never actually "flies around the world in one night", but rather has achieved a sort of teleporter mechanism that has allowed him to defeat The Law of Absolute Position and can "zap in and out" of a home in a quick instant, this device of course being able to move mass from one point to the next quick enough that the atoms that compose his mass never have a chance to move from the position they are in when he triggers the device(cause boy if it didn't, Santa would just be one big blob of guts and flesh all over your living room rug come Christmas morning).
Of course we can also theroise that Santa has perfected to an art form the ability to change from mass to energy and from energy to mass(E=mc^2 and M=e/c^2). This has been shown to happen in our universe, whereby mass can just appear from nowhere because of energy produced by cosmic background radiation and vice-versa: mass disappearing and becoming energy. Santa has some shaolin monk-like mind control over his body, that he can accelerate the atoms in his body and cause them to collide and convert him to an energy field, move to his next destination, slow his atoms down and re-appear as a body of mass, deliver his goods and repeat the process.
And, lastly, we could also theorise that Santa has a hellaciously big supply of knock out drops that he puts in the water supplies, we all drink it, go to sleep for weeks on end, and wake up thinking we've only slept for one night.
"Sure, it would be nice if Europeans agreed on a single language, but that's about as likely as Unix users agreeing on a single text editor." -Eloquence