I'm not so sure the "basic cable excuse" will fly anymore, when you've got 12 million people watching iCarly on Nickelodeon, 8 million watching Jersey Shore on MTV, or 7 million watching The Game on BET.
Adult Swim regularly gets a 1.0 in the demo, and it's on at the same time as Conan. You can't tell me more people get Cartoon Network than TBS.
And Conan got 4 million viewers on his first night, so clearly there are enough people who know how to find TBS on the dial. I think what happened is that a lot of people tuned in because of the hype, saw that Conan's show was pretty much an exact clone of his Tonight Show that they never watched, and never bothered to tune in again.