Big problem. I use Time.ly's All-in-One Event Calendar on a couple of WP sites I control and now when I login to wp-admin I get a message that 1.8.4 Premium is now available. OK. I click the Upgrade now link and as soon as I do that, my entire site goes 404 on me. I've had my web hosting company (LunarPages) reset this so I can get back in, but this doesn't explain why this is happening. And I'm on 3.5.
This is the link I get if I attempt the upgrade. I deleted my URL for purposes that noone tries to actually use this link or the 404 is likely to happen again.