crash on MacOS X

I'm trying to figure out a matplotlib crash on MacOS X.

When I try to plot anything the program exits with:
alloc: invalid block: 0xa08acb4: a 74 0

I first saw this with a program that uses the object interface, but then
I tried the simplest pylab demo I could find and the same thing happened.
- matplotlib installed from the universal package at
  configured to use numarray (1.5.1) and the TkAgg back end
- MacOS X 10.4.3 on a PPC mac
- ActiveState Aqua Tcl/Tk 8.4.13
- seen with two different versions of python:
  - ActiveState python 2.4.3 (PPC-only)
  - universal Python 2.4.3 (with modified to find my Tcl/Tk)

Any ideas?

-- Russell