Thank you for your very useful comments. I still feel that there must be more to it. It seems more natural for me to say "This is the book which I told you about" instead of "This is the book that I told you about" and "Here is the house which we looked at yesterday" instead of "Here is the house that we looked at yesterday". Could there be a formal/informal distinction here as well? These few examples and some others I have thought of lead me to guess that (in American English at least) "which" is freely used in restrictive clauses in informal speech.