I have lived in Scotland all my life (born 1949) and had never heard the term "plaid" being used to describe the pattern until relatively recently (perhaps around 1990). To me "plaid" was the garment and "tartan" was the pattern. It seems to be an American Term that has crept into use and is in danger of replacing the word "tartan".
user jimsun (August 2006)
I've always heard it pronounced plad, to rhyme with dad. It's the pronunciation I use myself.