pg:
"Thanks for applying. We'll respond by email about a week after the application deadline.
Please make a note of your username and password. You'll need them later to confirm you're coming."
I would think so too, except I didn't enter it anywhere, so I'm not sure how the association is made. Oh well, no sense worrying about such mundane details.
To apply, you need to sign in using your HN username and password. If you were already signed in, you probably just didn't notice that the form already knew who you were.
Er... what username and password?