I registered an account back in December, when I got my iPod. If my ipod is plugged into my comp when I go to the iTunes music store, my name appears, showing I'm logged.
So, after some looking around, I find a song (one by Joanna Newsom, for what it's worth). I enter the passcode from the Pepsi cap and click "BUY." Though I was already logged in, I was greeted with a prompt saying that I had to log in again. After doing so, I tried the above again, only to be told that the code I entered was already in use and could not be used again.
I suppose if I dicked around for another five or ten minutes, I would've figured out the problem. However, as I said earlier in this thread, the hassle wasn't worth it.