I have found a fairly significant issue with IE8 with regard to setting cookie expirations to 0 (so as to log a user out when they close the browser). It seems that each new tab or window is counted as a new session, so if a user opens a page on the site in a new tab/window, they have to login again unless they selected the option to stay logged in, which makes the cookie persistent and doesn't cause any problems. There has got to be a way to make this work like it should and always has in the past without forcing my users to stay logged in even after they close the browser. If it matters, I am setting the cookies from pHP like this:
setcookie("username",$username,0," ",".example.com");