If you have been getting the session errors as I have recently, it could be due to the Google IE Helper Chrome Frame. This is installed as part of Google Wave and goodness knows what else as I also run the Google Toolbar.
"Chrome frame adds itself to IE-user agent, so session cookies & some persistent cookies don't get recognized properly and causes errors."
I have verified that I did have this installed and I have disabled it, and touch wood, so far I haven't had any more session errors. (Okay so it's only been five minutes, but I was getting this constantly and it seems to have gone away now.)
To disable it in IE, Internet Options > Programs > Manage Addons > Toolbars and Extensions
Disable Google Chrome BHO.
Fingers crossed that this is the issue.
