Register: Username, Password, Hacker News Profile URL.

Click "forgot password" link.

Message: Modify your HN profile to include the phrase "RandomTextPleaseLetMeIn".

Click on "Verify me from HN".

The system lets you in. Now change your password.

Edit:

Instead of HN profile URL, you could also use Delicious ala 'save the following link with this tag "RandomFunk" to your Delicious profile'.

The goal is to prove to a website that has read-access something to which you have write-access.

This something is shared between you and the website before hand (at registration).