LGTM other than what Mark pointed out about this not applying to
subjects in the search buffer.

Quoth David Edmondson on Jan 25 at  1:48 pm:
> ---
>  emacs/notmuch-lib.el   |    6 ++++++
>  emacs/notmuch-print.el |    8 ++++++--
>  emacs/notmuch-show.el  |    5 ++++-
>  emacs/notmuch.el       |    5 +----
>  4 files changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/emacs/notmuch-lib.el b/emacs/notmuch-lib.el
> index 241fe8c..5b8a41c 100644
> --- a/emacs/notmuch-lib.el
> +++ b/emacs/notmuch-lib.el
> @@ -130,6 +130,12 @@ the user hasn't set this variable with the old or new value."
>    (interactive)
>    (kill-buffer (current-buffer)))
>  
> +(defun notmuch-prettify-subject (subject)
> +  (if (and subject
> +	   (string-match "^[ \t]*$" subject))
> +      (setq subject "[No Subject]"))
> +  subject)
> +
>  ;;
>  
>  (defun notmuch-common-do-stash (text)
> diff --git a/emacs/notmuch-print.el b/emacs/notmuch-print.el
> index 83eb525..51bb740 100644
> --- a/emacs/notmuch-print.el
> +++ b/emacs/notmuch-print.el
> @@ -19,6 +19,8 @@
>  ;;
>  ;; Authors: David Edmondson <dme@dme.org>
>  
> +(require 'notmuch-lib)
> +
>  (defcustom notmuch-print-mechanism 'notmuch-print-lpr
>    "How should printing be done?"
>    :group 'notmuch
> @@ -56,14 +58,16 @@ Optional OUTPUT allows passing a list of flags to muttprint."
>  
>  (defun notmuch-print-ps-print (msg)
>    "Print a message buffer using the ps-print package."
> -  (let ((subject (plist-get (notmuch-show-get-prop :headers msg) :Subject)))
> +  (let ((subject (notmuch-prettify-subject
> +		  (plist-get (notmuch-show-get-prop :headers msg) :Subject))))
>      (rename-buffer subject t)
>      (ps-print-buffer)))
>  
>  (defun notmuch-print-ps-print/evince (msg)
>    "Preview a message buffer using ps-print and evince."
>    (let ((ps-file (make-temp-file "notmuch"))
> -	(subject (plist-get (notmuch-show-get-prop :headers msg) :Subject)))
> +	(subject (notmuch-prettify-subject
> +		  (plist-get (notmuch-show-get-prop :headers msg) :Subject))))
>      (rename-buffer subject t)
>      (ps-print-buffer ps-file)
>      (notmuch-print-run-evince ps-file)))
> diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
> index e6a5b31..c602b3e 100644
> --- a/emacs/notmuch-show.el
> +++ b/emacs/notmuch-show.el
> @@ -985,7 +985,7 @@ buffer."
>        (notmuch-show-next-open-message))
>  
>      ;; Set the header line to the subject of the first open message.
> -    (setq header-line-format (notmuch-show-strip-re (notmuch-show-get-subject)))
> +    (setq header-line-format (notmuch-show-strip-re (notmuch-show-get-pretty-subject)))
>  
>      (notmuch-show-mark-read)))
>  
> @@ -1216,6 +1216,9 @@ Some useful entries are:
>  (defun notmuch-show-get-depth ()
>    (notmuch-show-get-prop :depth))
>  
> +(defun notmuch-show-get-pretty-subject ()
> +  (notmuch-prettify-subject (notmuch-show-get-subject)))
> +
>  (defun notmuch-show-set-tags (tags)
>    "Set the tags of the current message."
>    (notmuch-show-set-prop :tags tags)
> diff --git a/emacs/notmuch.el b/emacs/notmuch.el
> index 3f6b977..ce1e232 100644
> --- a/emacs/notmuch.el
> +++ b/emacs/notmuch.el
> @@ -440,10 +440,7 @@ Complete list of currently available key bindings:
>    "Display the currently selected thread."
>    (interactive "P")
>    (let ((thread-id (notmuch-search-find-thread-id))
> -	(subject (notmuch-search-find-subject)))
> -
> -    (if (string-match "^[ \t]*$" subject)
> -	(setq subject "[No Subject]"))
> +	(subject (notmuch-prettify-subject (notmuch-search-find-subject))))
>  
>      (if (> (length thread-id) 0)
>  	(notmuch-show thread-id