I. There are approximately two billion children (persons under 18) in the
world. However, since Santa does not visit children of Muslim, Hindu, Jewish
or Buddhist religions, this reduces the workload for Christmas night to 15%
of the total, or 378 million (according to the Population Reference Bureau).
At an average (census) rate of 3.5 children per house hold, that comes to
108 million homes, presuming that there is at least one good child in each.
II. Santa has about 31 hours of Christmas to work with, thanks to the
different time zones and the rotation of the earth, assuming he travels east
to west (which seems logical). This works out to 967.7 visits per second.
This is to say that for each Christian household with a good child, Santa
has around 1/1000th of a second to park the sleigh, hop out, jump down the
chimney, fill the stockings, distribute the remaining presents under the
tree, eat whatever snacks have been left for him, get back up the chimney,
jump into the sleigh and get on to the next house.
Assuming that each of these 108 million stops is evenly distributed around
the earth (which, of course, we know to be false, but will accept for the
purposes of our calculations), we are now talking about 0.78 miles per
household; a total trip of 75.5 million miles, not counting bathroom stops
or breaks. This means Santa's sleigh is moving at 650 miles per second 
3,000 times the speed of sound. For purposes of comparison, the fastest
manmade vehicle, the Ulysses space probe, moves at a poky 27.4 miles per
second, and a conventional reindeer can run (at best) 15 miles per hour.
III. The payload of the sleigh adds another interesting element. Assuming
that each child gets nothing more than a medium sized Lego set (two pounds),
the sleigh is carrying over 500 thousand tons, not counting Santa himself.
On land, a conventional reindeer can pull no more than 300 pounds. Even
granting that the "flying" reindeer could pull ten times the normal amount,
the job can't be done with eight or even nine of them Santa would need
360,000 of them. This increases the payload, not counting the weight of the
sleigh, another 54,000 tons, or roughly seven times the weight of the Queen
Elizabeth (the ship, not the monarch).
IV. 600,000 tons traveling at 650 miles per second creates enormous air
resistance  this would heat up the reindeer in the same fashion as a
spacecraft reentering the earth's atmosphere. The lead pair of reindeer
would absorb 14.3 quintillion joules of energy per second each. In short,
they would burst into flames almost instantaneously, exposing the reindeer
behind them and creating deafening sonic booms in their wake.
The entire reindeer team would be vaporized within 4.26 thousandths of a
second, or right about the time Santa reached the fifth house on his trip.
Not that it matters, however, since Santa, as a result of accelerating from
a dead stop to 650 m.p.s. in .001 seconds, would be subjected to centrifugal
forces of 17,500 g's. A 250 pound Santa (which seems ludicrously slim) would
be pinned to the back of the sleigh by 4,315,015 pounds of force, instantly
crushing his bones and organs and reducing him to a quivering blob of pink
goo.
Therefore, if Santa did exist, he's dead now.

