We plan to release significantly updated and improved versions of both the Winforms and ASP.NET Report Viewer controls in Visual Studio 2010.  This is going to include the long awaited local mode support for the new report processing engine, originally released with SQL Server Reporting Services 2008.  Most importantly, this provides RDL 2008 features (e.g. tablix, enhanced charts, gauge, rich text) in local mode without connection to a report server.  Furthermore, a number of other new features are coming as well in the Report Viewer controls.  Related to this, my esteemed colleague Brian Hartman started blogging recently.  His blog is definitely worth keeping an eye on regarding common questions on current and future versions of the Report Viewer.

FAQ: What is the current level of RDL support in Visual Studio 2008 Report Viewer controls? 

If you are using local mode, you probably already noticed that attempting to load RDL 2008 based reports results in the following error:

The report definition is not valid.
Details: The report definition has an invalid target namespace 'http://schemas.microsoft.com/sqlserver/reporting/2008/01/reportdefinition' which cannot be upgraded.

You cannot use RDL 2008 features in VS 2005 or VS 2008 report viewer controls in local mode, because the controls are using the same report processing engine that was shipped with SQL Server 2005 (supporting only the RDL 2005 namespace and feature set).  As a side-note, VS 2008 shipped almost 6 months before SQL Server 2008 became available.

If you want to use RDL 2008 features with the report viewer controls available todayserver mode is your only viable option, because report processing is performed remotely on a Reporting Services 2008 server.  Please check Brian's blog posting about RS 2008 and the Report Viewer controls for more details.  A general overview of the differences in functionality between Report Viewer and RS 2008 is available in the documentation as well.