How to make IPython notebook matplotlib plot inline

I am trying to use IPython notebook on MacOS X with Python 2.7.2 and IPython 1.1.0.

I cannot get matplotlib graphics to show up inline.

import matplotlib
import numpy as np
import matplotlib.pyplot as plt
%matplotlib inline  

I have also tried %pylab inline and the ipython command line arguments --pylab=inline but this makes no difference.

x = np.linspace(0, 3*np.pi, 500)
plt.plot(x, np.sin(x**2))
plt.title('A simple chirp')
plt.show()

Instead of inline graphics, I get this:

And matplotlib.get_backend() shows that I have the 'module://IPython.kernel.zmq.pylab.backend_inline' backend.