As first mentioned in this thread: http://boards.straightdope.com/sdmb/showpost.php?p=13965443&postcount=95
it is claimed that “I Can’t Believe It’s Not Butter” does, indeed, contain butter.
A look at the ingredients of “I Can’t Believe It’s Not Butter” shows no butter, but does show buttermilk.
So, is buttermilk equivalent to butter? It doesn’t sound like it to me.