A past episode of BrainStuff – HowStuffWorks explains that odd, sorta-Brit/sorta not accent that some American actors, politicians and other notable names used to affect. What we're hearing is known as the Transatlantic (Mid-Atlantic) Accent, which was taught to actors, media types and students alike, mostly prior to the second world war.
What's funnier than the accent itself is the way some actors spoke that way consistently throughout their early films and then dropped it in their later years, thus rendering their voices practically unrecognizable. Mia Farrow in, say, Rosemary's Baby vs. her later films comes to mind. Via Laughing Squid