I saw this thread then I saw the story on the Drudge Report. It piqued my interest so I decided to check it out.
It was bizarre, that's for sure. It was fun watching Letterman squirm uncomfortably.
It's possible Joaquin is suffering a nervous breakdown. One can tell quite a bit about a person's state of mind simply based on their physical appearance. The unkempt beard, lack of affect in his voice, shaggy hair, and dark glasses are dead giveaways. A combination of overwork and personal problems could could push anyone over the edge. It's also possible he was intoxicated, which may or may not be related to a breakdown.
If it was a stunt, it was well-executed and it paid off. Joaquin is an actor after all. I choose to not own a television, but I sought out the segment online and watched it on the CBS website.
Whatever it was, it's getting attention.