You're right, the content is properly protected when I clear my cookies. Sorry about that. There still might be an issue, however.
Check out http://blog.schulte.org/archives/2005/01/01/this-should-be-protected/ and use 123 as the password. Notice at the bottom that it's not possible to add comments. Only "This post is password protected. Enter the password to view comments." is shown.
If I revert back to the default theme, that same page shows the comment block fine (when the password is entered), and I am able to post a response. Then when I switch back to Kubrick 1.3.0-rc2 the comments are gone and the same "This post is password protected. Enter the password to view comments." message is shown even when I can see the actual content fine.
A problem with the distributed Kubrik theme?