If the "using namespace" directive is used with Qt specific code as exemplified below, the application doesn’t compile with MSVS2008 on Windows.
This behavior is reproduced on Windows Vista.

#include <QtGui>

namespace A
{

class AA
{
};
};

namespace B
{
using namespace A;
class BB : public QObject

{ Q_OBJECT signals: /* if we make foo as simple member function in spite of signal it complies*/ void foo(AA *a); }

;
};

#include "main.moc"
using namespace B;
int main()
{

BB b;
return 0;
}

It compiles without error on Linux and MSVS2005.