Ok, it's wired, look following example:
Qt Code: 
  1. #include <QtGui>
  2.  
  3. class myItem : public QGraphicsRectItem
  4. {
  5. public:
  6.     myItem(QGraphicsItem *parent = 0) : QGraphicsRectItem(parent)
  7.     {
  8.         setAcceptHoverEvents(true);
  9.     }
  10.  
  11.     void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget = 0)
  12.     {
  13.         if (option->state.testFlag(QStyle::State_MouseOver))
  14.             setBrush(QColor(0,230,230));
  15.         else
  16.             setBrush(QColor(230,230,230));
  17.         QGraphicsRectItem::paint(painter, option, widget);
  18.     }
  19. };
  20.  
  21. class myItem2 : public QGraphicsTextItem
  22. {
  23. public:
  24.     myItem2(QGraphicsItem *parent = 0) : QGraphicsTextItem(parent)
  25.     {
  26.         setAcceptHoverEvents(true);
  27.     }
  28.  
  29.     void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget = 0)
  30.     {
  31.         painter->setBrush(QColor(230,230,230));
  32.         if (option->state.testFlag(QStyle::State_MouseOver))
  33.             painter->setBrush(QColor(0,230,230));
  34.         painter->drawRect(option->rect);
  35.         QGraphicsTextItem::paint(painter, option, widget);
  36.     }
  37. };
  38.  
  39. class myItem3 : public QGraphicsSimpleTextItem
  40. {
  41. public:
  42.     myItem3(QGraphicsItem *parent = 0) : QGraphicsSimpleTextItem(parent)
  43.     {
  44.         setAcceptHoverEvents(true);
  45.     }
  46.  
  47.     void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget = 0)
  48.     {
  49.         painter->setBrush(QColor(230,230,230));
  50.         if (option->state.testFlag(QStyle::State_MouseOver))
  51.             painter->setBrush(QColor(0,230,230));
  52.         painter->drawRect(option->rect);
  53.         QGraphicsSimpleTextItem::paint(painter, option, widget);
  54.     }
  55. };
  56.  
  57. int main(int argc, char **argv)
  58. {
  59.     QApplication app(argc, argv);
  60.  
  62.     myItem it;
  63.     it.setRect(0,0,30,30);
  64.     scene.addItem(&it);
  65.  
  66.     myItem2 it2;
  67.     it2.setPlainText("test!");
  68.     scene.addItem(&it2);
  69.     it2.setPos(0,40);
  70.  
  71.     myItem3 it3;
  72.     it3.setText("test2!");
  73.     scene.addItem(&it3);
  74.     it3.setPos(0,80);
  75.  
  76.     QGraphicsView view(&scene);
  77.     view.show();
  78.  
  79.     return app.exec();
  80. }
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myItem and myItem3 behaves like expected, myItem2 not. Even if you set various values for setTextInteractionFlags(). So I came to the conclusion it does not work for QGraphicsTextItem. The funny thing is that you can read in the docs to QGraphicsTextItem:
Note: QGraphicsTextItem accepts hover events by default. You can change this with setAcceptHoverEvents().
Hm, I guess that the above behavier is not intended. So it seems to be a bug special for QGraphicsTextItem. If nobody speaks against, I'll report it the next days...