In the past, I've used the runas commandline to launch another instance if IE on a secondary account for testing, since our in-house web-app application uses windows authentication to identify a user. With the new multi-process model if IE8, this doesn't seem to work. Has anyone seen a workaround for it?
See this: http://blogs.msdn.com/ie/archive/2009/05/06/session-cookies-sessionstorage-and-ie8.aspx for info on session merging.
As an aside: RunAs is currently broken with IE8 on XP, but works on Vista. The IE team is aware/investigating this issue.