have you checked in IE8 to see if it is actually accepting/rejecting the session cookie?
have you tested other versions of IE?
can you get a basic session to work on IE8 (e.g., just start a session, save a random variable, and print it on the next page)?
I suspect that in order to help further, we'd need to see all the code up to your [font="monospace"]if(isset($_SESSION['sessionname']))[/font], and probably the code that started the session as well.