In IE6/7 if you open 2 independent browsers (not using “New Window” or “New Tab”) they do not share common session and you can login to, let’s say, a web mail account under 2 different users at the same time.
This is not the case with IE8. Using 2 browser (and seeing 2 “iexplore.exe” processes in task manager) if you try to login as 2 users – the second will take over the first. I guess it’s a new “feature” of the browser.
But IE8 gives you a way out, albeit not obvious one. In menu, you can select File -> New Session. This will open a new browser window with independent session.