great overview so far. I'm using python (specifically scipy + matplotlib) as a matlab replacement since 3 years working at University. I sometimes still go back because I'm familiar with specific libraries e.g. the matlab wavelet package is purely awesome.
I like the http://enthought.com/ python distribution. It's commercial, yet free for academic purposes and, as far as I know, completely open-source. As I'm working with a lot of students, before using enthought it was sometimes troublesome for them to install numpy, scipy, ipython etc. Enthought provides an installer for Windows, Linux and Mac.
Two other packages worth mentioning:
ipython (comes already with enthought) great advanced shell. a good intro is on showmedo http://showmedo.com/videotutorials/series?name=PythonIPythonSeries
nltk - the natural language toolkit http://www.nltk.org/ great package in case you want to do some statistics /machine learning on any corpus.