Failed to load expression host assembly. Details: Could not load file or assembly ...

 In Reporting Services, if you are getting this error is because you need to copy your referenced assemblies of the report into the folder

 Program Files\Microsoft SQL Server\MSSQL.3\Reporting Services\ReportServer\bin 

If you are in the designer you need to copy it into this one too

 Program Files\Microsoft Visual Studio 8\Common7\IDE\PrivateAssemblies

http://support.microsoft.com/kb/920769