I believe the possum is the southern cousin of the opossum. They are typically found only in the region known as the deep south.
One of the more distinct differences is that while the opossum will play dead if threatened (such as starting your car in an attempt to run it over) - the possum will actually die. It just isn't all there and therefore doesn't deserve the silent vowel.
Of course I could be wrong here and it could be just a simple drop of the O for convience.... we are talking about a state where the people use two words (over yonder) to describe any distance between 20 feet and 20 miles.... hmmm?
