As well as the already-mentioned Garbo, studio bosses decided Karloff was well enough known that only his (assumed) name need be used on posters. I think they did the same with Lugosi, too.

BTW, on the horror movie theme, I think that Nosferatu (in George's message, 01 Dec 07 - 06:34 AM) refers to a date in the calendar, rather than the character. The vampire in that film was called Graf Orlock.