I started a thread about teeth clipping on River Cottage forum, it generated lots of replies!!
But I decided not to, yes their teeth are sharp and the sow does get a few scratches and a little sore, but she is free to move around as she pleases, commercially if the are kept in farrowing crates maybe they feel its best for the sow to clip piglets teeth?
(As an aside Uddermint cream is great for soothing sore teats in pigs!)