Thank you for your replies Peter and Jason.
I'm afraid I'll now have to read the manual chapter dealing with faces as the few things I ever knew about them has vanished from my memory a long time ago :)
>This variable has been obsolete since Emacs 20, stopped working in Emacs 23 (or
>maybe even 22.something) and was removed about a year ago for 24.1.
It appears that it was not completely removed in 24.1 as it was still working for me until 24.2 included.
Not that this observation changes anything to the fact that a rework of my .emacs was long overdue.