QWidget::mouseMoveEvent not firing when cursor over child widget

The problem with your code is that you need to enable mouse tracking for all widgets explicitly. You can do this by iterating over all children of your main widget, and calling setMouseTracking(True) for each of them. Here I've overridden setMouseTracking() to do just that:

import sys
from PyQt4 import QtCore, QtGui

class MyWindow(QtGui.QWidget) :
    def __init__(self):
        QtGui.QWidget.__init__(self)
        tabs = QtGui.QTabWidget()
        tab1 = QtGui.QWidget()
        tab2 = QtGui.QWidget()
        tabs.addTab(tab1, "Tab 1")
        tabs.addTab(tab2, "Tab 2")
        layout = QtGui.QVBoxLayout()
        layout.addWidget(tabs)
        self.setLayout(layout)
        self.setMouseTracking(True)

    def setMouseTracking(self, flag):
        def recursive_set(parent):
            for child in parent.findChildren(QtCore.QObject):
                try:
                    child.setMouseTracking(flag)
                except:
                    pass
                recursive_set(child)
        QtGui.QWidget.setMouseTracking(self, flag)
        recursive_set(self)

    def mouseMoveEvent(self, event):
        print 'mouseMoveEvent: x=%d, y=%d' % (event.x(), event.y())


app = QtGui.QApplication(sys.argv)
window = MyWindow()
window.setFixedSize(640, 480)
window.show()
sys.exit(app.exec_())