The following links may answer your question:
http://blogs.msdn.com/ie/archive/2009/05/06/session-cookies-sessionstorage-and-ie8.aspx
http://www.velocityreviews.com/forums/t363555-session-being-kept-from-brower-to-browser.html
http://www.eggheadcafe.com/software/aspnet/34215957/ie-8-release-version-is-s.aspx
http://www.techieon.com/2009/07/differences-of-session-sharing-behavior-in-ie7-ie8/
The search phrase I used in google was "internet explorer 6 shared session"
-- Sriram