If you include <designeditor.hpp> and try to use TPropertyEditor in C++ Builder, you'll run into BCC32 errors complaining about multiple declaration for 'IPropertyDescription' and ambiguity between 'IPropertyDescription' and 'Designintf::IPropertyDescription'. This is true for every version post-BCB6, including the latest CB2010.
The namespace ambiguity problem is an inherent problem with C++ Builder because every HPP file that is generated from Delphi includes the namespace in the header file. We all know that's *BAD* now, but it's a decision that dates back to the first version where even the std namespace was implicit included. Now that we've found ourselves too deep in the rabbit hole, there's really no easy way out as far as backward compatibility is concerned.
But, for this particular problem, there's a solution.
Before you include DesignEditors.hpp, you should first include PropSys.hpp, such as,
class PACKAGE TMyComponentEditor: public TPropertyEditor
Perhaps the better way would be for DesignEditors.hpp to include PropSys.hpp at the very top of the file, so anyone who uses DesignEditors.hpp doesn't need to remember including PropSys.hpp explicitly. That one's for Embarcadero to decide.