We are using Reporting Services 2005 on SQL 2005 and when a report is running then Reporting Services seems to block for that user. e.g. navigating in /Reports/Pages/Folder.aspx will wait until the report finishes before displaying anything. I have had a look at the rsreportserver.config file but nothing in there seems to be limiting things. Also it doesn't seem to be IIS that is blocking.
If the answer to this is Upgrade to 2008 than that is fine.