http://meadhall.homestead.com/BoarsPigsandMyth.html
To the modern mind the uncleanness of the pig is obviously connected to the pig's affinity for dirt: on the one hand, a demonic trait; on the other, at least to our minds, an unhygenic one. To the ancients the concept was more ambiguous. The primary meaning of uncleanness was holiness. Therefore, to come in contact with an unclean creature, that is, a creature highly charged with spiritual power, was somewhat equivalent to touching a radioactive object--such an object as the Ark of the Covenanant, for instance.
The attitude of the Jews toward pigs, however, was not strikingly different than that of other peoples in the ancient Near East. The Syrians neither sacrificed nor ate pigs, and if a man touched a pig he was unclean for the rest of the day (Frazer. 546). Among the Egyptians, also, touching a pig was unclean, and swineherds were a class almost of untouchables, forbidden even to enter a temple (Frazer. 548). Once a year, however, pigs were sacrificed to Osiris and to the moon, and at that time their flesh was eaten.
Thus, the eating of pork, at the proper time, was a sacremental act. There is in myth a tendency for things to mean, or to be, also their opposite; the pig's very holiness makes it unclean. Another example of the pig's holiness is the fact that Jews were forbidden to kill pigs as well as to eat them.
With the Hebrews, however, as their religion grew more anthromorphic and more transcendant, the holiness of the pig lost its roots, while the sense of uncleanness remained, a sense reinforced by the affinity of the pig for mud. It remained for Christianity and Peter's dream (Acts 10. 9-16) for the next logical step, the demotion of the pig from unclean animal to simply one of God's creatures placed here for the benefit of mankind.