In Q3 2010 we introduced native Silverlight print, which is set as default. However the Silverlight document print has very limited API which does not allow for us to specify the orientation. Thus for in browser applications we have exposed ReportViewer.UseNativePrinting
property for the Silverlight viewer - please set it to false. This will force the Silverlight Report Viewer to revert to the previous printing behavior using Adobe's PDF plug-in. Note however that this would not work for out of browser applications.
The other option as you've pointed out is manually setting the page orientation from the print dialog.
the Telerik team