I've seen this particular piece of writing every year for several years now. Much like the Cartoon laws of physics, it makes it's rounds via email, and ends up in practically everyone's mailbox eventually. After a simple search it can be found on the web quite easily. Normally I wouldn't bother to node something like this, but someone asked about it in the CBox the other day, they claimed to have discovered a way to work around Santa's problem of combusting reindeer by compensating with a larger number of more expendable reindeer.
I did a search and couldn't find any evidence that it had been noded before, and so I decided to add it to the lexicon. The original Author seems to be unknown, and I wouldn't be surprised to find out that this was passed between professors and grad students on blurry sheets of carbon copy long before the internet became so popular. However, if you know who authored this please /msg me and I will add a citation.
 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 (except maybe in Japan) 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 household, that comes to 108 million homes,
presuming that there is at least one good child in each.

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
second3,000 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.

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 themSanta 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).

600,000 tons traveling at 650 miles per second creates enormous air
resistancethis 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
acceleration 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. Merry Christmas.