I was just looking at the iso-contouring code of Zach Pincus (http://mail.scipy.org/pipermail/scipy-user/2009-July/021719.html), and saw it is implemented with the marching squares algorithm. On the task list that algorithm is listed as well, with a note to investigate patent issues. So I had a look at that.
The patent issue was earlier raised in this thread in 2004: http://osdir.com/ml/python.matplotlib.devel/2004-10/msg00066.html. The open question was whether marching squares was covered under the patent for marching cubes or not. This is a 1985 patent and has expired in the meantime, as noted here: http://en.wikipedia.org/wiki/Marching_cubes.
IANAL, but I think there are no patent issues anymore.
Separate question to Zach: the code you sent to the SciPy list in July was GPL licensed. You noted you were willing to send it again under a different license. Could you please send it to the list with a BSD/MIT/similar license?