Up until Half Blood Prince, Voldemort's name COULD be spoken but people were so terrified of him that they were afraid to say the name aloud. However, in Deathly Hallows, the word had a Taboo curse placed on it. This meant that anyone who spoke the name aloud could immediatly be found by snatchers. This curse was placed because usually only the bravest & most defiant Voldemort protesters e.g. members of the Order Of The Phoenix, would dare to say the name out loud. This meant that the snatchers could easily round up the anti-Voldemort protesters & apply the appropriate punishments e.g. Sentences in Azkaban etc.