I can't speak officially for anyone at YC, but I happen to know that they have been experimenting with nginx 1.4 with SPDY support and there was an incompatibility with the Arc web server and specifically with cookie handling.
An intelligent guess says this has something to do with what you are seeing.
I can confirm that there was a technical change on HN today that affected other browsers. I have been just turning on my browser (Chrome running on Windoze) and coming over to HN already logged in for months now. Just today I had to log in again, which because HN's new emailed link for password updates allowed me to specify a password, has let me choose a memorable password specific to HN. (The former way HN updated passwords by email was to send a new, random password to my email address, as for example when I logged in via a different computer without an HN cookie set.) So far, so good.
An intelligent guess says this has something to do with what you are seeing.