The simplest "program" is two lines long -- the problems are related to matplotlib only:
import matplotlib.pyplot as plt

I set the environment variable TM_PYTHON to /sw/bin/python
In Terminal, "which ipython" returns /sw/bin/ipython
If I execute both lines in ipython, the matplotlib window pops up, as expected.
While my two-line script is running in TextMate, 
ps -Am
returns two (why two?) python processes:
 1835 ??         0:00.83 /sw/bin/python -u /Users/claushaslauer/Desktop/present work/test/
 1812 ??         0:00.89 /sw/bin/python -u /Users/claushaslauer/Desktop/present work/test/

sampling those two processes results in essentially identical files, only difference is the number in front of each line. I attached both files, because I can't interpret them.

For matplotlib:
In [3]: matplotlib.__version__
Out[3]: '1.0.svn'
installed via

Does this point to any issue?