He's also filming a documentary about his music venture. So maybe the Letterman show was an act for his documentary, in which case he's doing a great job acting. But with the itching, fidgeting, and drowning moments of lost consciousness amidst an ongoing interview, he's gotta be on something...
This interview made me uncomfortable. But Dave seemed to have a field day with it.
.saint edit, found one
PRIME_BBCODE_SPOILER_SHOW http://www.youtube.com/v/HXpYk7WGN5Y