What web browser are those of you who are having problems using? It may be an issue with how your browser is handling the session cookie that keeps you logged in. I'm not sure what updates they did last night but it could have effected the way your browser deals with the site.
I am using Firefox and am running version 3.6 and I have no problem staying logged in even over multiple tabs.
So do some of you guys stay logged in over an extended period of inactivity normally? For example, before today if you logged in and went to bed would you still be logged in when you woke up?
I have my browser set to expire cookies and treat all cookies like session cookies so after a period of inactivity, usually about 10 minutes, I am logged out automatically anyway.