The term Sin Eater does sound like the title of a dodgy horror film. In fact perhaps it is. I wouldnt know. But I came across it in a news item on the BBC last month (September 2010)
It is a fascinating idea and perhaps you will not be surprised to learn that this ancient tradition survived in the east of Wales and just over the border in Shropshire and Herefordshire longer than anywhere else. Indeed, it was still practiced into the early 20th century.
It is a bizarre adoption process I suppose. After a death someone would be paid to eat and drink over the body. As a result of the ritual the sin eater would take on the sins of the dead person and their soul would then be able to rest, free of sin. The church wasnt that keen on the idea but often the local vicar would turn a blind eye in order to keep his parishioners happy.
Often the ritual was performed by a beggar, although some villages had a resident sin-eater. They would turn up at the bedside, where a relative would place a crust of bread on the chest of the dying and pass a bowl of beer to him. I imagine that if you thought you were just a bit under the weather and the sin eater was lurking in the background, waiting for a snack, you would start to worry. Anyway, after praying or reciting the ritual, he would then drink and eat the bread, thus adopting the sins of the dying.
As I said, it was mentioned on the BBC in connection with the grave of Richard Munslow who died in Ratlinghope in 1906. The grave stone has recently been restored since he was a well known farmer in the area who had a second career as a sin eater, munching on scraps of bread whilst others squabbled about the inheritance. I would have thought that for those with rather more interesting lives, a three course meal would have been more appropriate than a dry crust in order to absolve them of sin, but perhaps I am being unkind. However, I have to say it is odd to think that this sort of thing was going on in the lifetime of my grandparents.
I end this piece with this passage by B.S. Puckle in a book called Funeral Customs (1926) which goes to show how odd people can be.

"Professor Evans of the Presbyterian College, Carmarthen, actually saw a sin-eater about the year 1825, who was then living near Llanwenog, Cardiganshire. Abhorred by the superstitious villagers as a thing unclean, the sin-eater cut himself off from all social intercourse with his fellow creatures by reason of the life he had chosen; he lived as a rule in a remote place by himself, and those who chanced to meet him avoided him as they would a leper. This unfortunate was held to be the associate of evil spirits, and given to witchcraft, incantations and unholy practices; only when a death took place did they seek him out, and when his purpose was accomplished they burned the wooden bowl and platter from which he had eaten the food handed across, or placed on the corpse for his consumption"

A bit mean when all is said and done. You provide a valuable service and this is how you are treated. No wonder that as a career option it never really caught on.