I gave it a quick try and it seems to work at least for a top level widget:
Qt Code: 
  1. #include <QtGui>
  2.  
  3. class Window : public QWidget
  4. {
  5. 	Q_OBJECT
  6. public:
  7. 	Window()
  8. 	{
  9. 		int spacing = 20;
  10.  
  11. 		QSpinBox* spinbox = new QSpinBox(this);
  12. 		spinbox->setStyleSheet("background: red");
  13. 		spinbox->setValue(spacing);
  14.  
  15. 		QLabel* label = new QLabel(this);
  16. 		label->setStyleSheet("background: blue");
  17. 		label->setNum(spacing);
  18.  
  19. 		QVBoxLayout* layout = new QVBoxLayout(this);
  20. 		spacer = new QSpacerItem(spacing, spacing);
  21. 		layout->addWidget(spinbox);
  22. 		layout->addItem(spacer);
  23. 		layout->addWidget(label);
  24.  
  25. 		connect(spinbox, SIGNAL(valueChanged(int)), label, SLOT(setNum(int)));
  26. 		connect(spinbox, SIGNAL(valueChanged(int)), this, SLOT(setSpacing(int)));
  27. 	}
  28.  
  29. private slots:
  30. 	void setSpacing(int spacing)
  31. 	{
  32. 		spacer->changeSize(spacing, spacing);
  33. 		layout()->invalidate();
  34. 	}
  35.  
  36. private:
  37. 	QSpacerItem* spacer;
  38. };
  39.  
  40. int main(int argc, char* argv[])
  41. {
  42. 	QApplication a(argc, argv);
  43. 	Window w;
  44. 	w.show();
  45. 	return a.exec();
  46. }
  47.  
  48. #include "main.moc"
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I suppose the spacing is not homogeneous over the layout? Otherwise you could simply use QBoxLayout::setSpacing().