Tooltips not showing up on Ubuntu using PyQt4

Question

I'm getting started with PyQt4 and tested the following code...

import sys
from PyQt4 import QtGui
from PyQt4 import QtCore


class Tooltip(QtGui.QWidget):
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)

        self.setGeometry(300, 300, 250, 150)
        self.setWindowTitle('Tooltip')

        self.setToolTip('This is a <b>QWidget</b> widget')
        QtGui.QToolTip.setFont(QtGui.QFont('OldEnglish', 10))


app = QtGui.QApplication(sys.argv)
tooltip = Tooltip()
tooltip.show()
sys.exit(app.exec_())

However, no tooltip shows up! I'm on Ubuntu 10.04. I also tried an icon and that didn't work either.