It has been my understanding that the snake of christian mythology was an angel called Satani'el. It was this beings belief that mankind should be self aware, should have freedom of choice, unlike the angels themselves. The "forbidden knowledge" was as such, said self awareness, I.e. the modesty that caused adam and eve to cover themselves with leaves. It is also possible that this was when mans "sin nature" came into being. A snake can also represent cycles, evident in the symbol of Oroboras the snake that is devouring itself.