I would like to have diary entries of the form

Thu, Apr 5, 2007 Today's entry

in which the string "Thu, Apr 5, 2007" has diary face.  This
fontification is not provided by the default diary setup, but I had
hoped and expected to get it by customizing diary-date-forms
appropriately.  However, after making what I think is the necessary
customization, I still don't get the fontification I want.  I can
reproduce the problem with emacs -Q:

1. Let ~/.emacs consist only of this sexp: