When using implicit styles mechanism and you merge Telerik.Windows.Controls.Input an exception is thrown.

If the XAML file is set with BUILD ACTION PAGE the issue is not reproducible.

Another workaround is to reference dlls only, not xaml theme files.

Issue is reported in several msdn / stackoverflow threads and it appears to be an MS-TextBox one. 
http://stackoverflow.com/questions/6850713/weird-xaml-parsing-error-when-trying-to-set-textbox-isreadonly