I had this same problem. Installed
matplotlib-1.0.0-python.org-py2.6-macosx10.4.dmg on 15 Sept 2010
from matplotlib.sourceforge.net on OSX 10.6.1 and python 2.6 from python.org, and got same error when trying to plot(something). It turns out that all the image files (for buttons on the plot window, etc) in that directory have permissions similar to
-rw--w---- instead of -rwxr-xr-x.
In our case, the lab administrator account (say, “labadmin”) has administrator privileges and was used to give permission to run the installer. However, the files end up being owned by “macadmin”, and labadmin isn’t able to chmod or chown them. Otherwise, it would have been an easy problem to solve.
I just checked the bug tracker and didn’t see this error listed.