I notice that only when running IE8 in compatibility view for the client support portal does the service cases grid display correctly. Has one of the CRM rollup updates addressed this issue, making CMR4 compatible with IE8?
Please see this post http://blogs.msdn.com/b/crm/archive/2009/05/07/microsoft-dynamics-crm-and-ie8-tweak.aspx
Just if you have any add-ons which is causing the problem. If its is try disabling the add-on and see if still there is an error.
Hope this helps.