1) No known species of reindeer can
   fly.  BUT, there're 300,000 species
   of living organism yet to be
   classified, and while most of these
   are insects and germs, this does
   not COMPLETELY rule out flying
   reindeer, which only santa has ever
   seen.
 
2) There are 2 billion children
   (persons under 18) in the world.
   BUT, since Santa doesn't (appear)
   to handle Muslim, Hindu, Jewish,
   and Buddhist children, that reduces
   the workload to 15% of the total
   - 378 million according to the
   population reference bureau.  At
   an average (census) rate of 3.5
   children perhousehold, thats 91.8
   million homes.  One presumes
   there's at least one good child in
   each.
 
3) Santa has 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 822.6 visits per
   second.  This is to say that for
   each Christian household with good
   children, Santa has 1/1000th of a
   second to park, hop out of the
   sleigh, jump down the chimney, fill
   the stockings, distribute the
   remaining presents under the tree,
   eat whatever snacks have been left,
   get back up the chimney, get back
   into the sleigh, and move onto the
   next house. Assuming that each of
   these 91.8 million stops are
   evenly distributed around the Earth
   (which, of course, we know to be
   false, but for the purposes of our
   calculations we will accept), we
   are now talking about .78 miles per
   household, a total trip of 75-1/2
   million miles, not counting stops
   to do what most of us do at least
   once every 31 hours, plus feeding,
   etc.
 
   This means that Santa's sleigh is
   moving at 650 miles per second,
   3,000 times the speed of sound.
   For purposes of comparison, the
   fastest man-made vehicle on Earth,
   the Ulysses space probe, moves at a
   poky 27.4 miles per second - a
   conventional reindeer can run,
   tops, 15 miles per hour.
 
 
4) The payload on the sleigh adds
   another interesting element.
   Assuming that each child gets
   nothing more than a medium-sized
   lego set (2 pounds), the sleigh
   is carrying 321,300 tons, not
   counting Santa, who is invariably 
   described as overweight.  On land,
   conventional reindeer can pull no
   more than 300 pounds. Even
   granting that "flying reindeer"
   (see point #1) could  pull TEN
   TIMES the normal amount, we cannot
   do the job with eight, nor even
   nine.  We need 214,200 reindeer.
   This increases the payload - not
   even counting the weight of the
   sleigh - to 353,430 tons.  Again,
   for comparison, this is four times
   the weight of the Queen Elizabeth
   (the Ship).
 
 
5) 353,000 tons travelling at 650
   miles per second creates enormous
   air resistance - this will heat the
   reindeer up in the same fashion as
   spacecraft re-entering the Earth's
   atmosphere.  The lead pair of
   reindeer will absorb 14.3
   QUINTILLION joules of energy.
   Per second.  Each.  In short, they
   will burst into flame almost
   instantaneously, exposing the
   reindeer behind them, and create
   deafening sonic booms in their
   wake.  The entire reindeer team
   will bevaporized within 4.26
   thousandths of a second.  Santa,
   meanwhile, will be subjected to
   centrifugal force against the back
   of his sleigh equal to 4,315,015
   pounds of force.  In conclusion,
   If Santa ever DID deliver presents
   on Christmas Eve, he's dead now.