I'm not aware of any tool or work around for this problem and given the amount of similar or related questions elsewhere (e.g. MSDN Forums) without any satisfactory answers I doubt something exist.
Of course you could always go ahead and write a XSL Transformation from the new 2008 schema to the old 2005 schema, but given the amount of changes and especially extensions this would still be a tedious task with quite some possible loss of information, even while completely ignoring tablix.
Depending on your requirements you might be able to sit this out until Visual Studio 2010 is released, which is supposed to feature updated ReportViewer controls with on par support for the new 2008 schema. Unfortunately this release has yet see the light though, at least they haven't made it in time for beta 1, see 'Where’s the new Report Viewer?'.