On an ubuntu 10.10 box:

longs[~]> nosetests matplotlib
The CococaAgg backend required PyObjC to be installed!
  (currently testing v1.3.7)

Note that starting the tests differently does work:

longs[junk]> nosetests -vvs matplotlib.tests
matplotlib.tests.test_axes.test_arc_ellipse.test ... FAIL
matplotlib.tests.test_axes.test_arc_ellipse.test ... ok
etc..

ps - mpl should leave all its tests results in a temporary directory. Right now it leaves a massive amount of junk in whatever directory the test suite was called. If an unsuspecting user attempts to do that in a directory with lots of other data, disentangling the resulting mess could take a while.