For certain configurations of (x,y) points, matplotlib.delaunay.delaunay() produces illegal triangulations - in particular they contain overlapping triangles.
As a consequence, the "Triangulation" class cannot be used for such data, as it fails during construction (overlapping triangles break the assumptions used for the convex-hull computation).
A sample dataset is available at https://dl.dropbox.com/u/6960989/bad_tri/points.csv
The attached image is zoomed-in on some overlapping triangles in the resulting triangulation.

(will try to attach some code later)