It seems that the ASP.net report viewer requires session state enabled. This is a major limitation with the integration with SP2010 given that session state is disabled by default.
In order to be able to use Telerik Reporting on a SP2010 site it seems that we need to enable session state in the entire web application (instead of just enabling it on the page with the report viewer), which in my opinion, is not an acceptable solution. Do you have any plans to remove the need to use session state in future releases?
It would also be a good idea if you clearly state this limitation in your literature so people are aware of it before making a purchase decision. You have a page that describes how to integrate the report viewer with SharePoint
but you do not mention anything about the session state limitation which I think can by very misleading.