[Bf-python] Script newbie needs help with per-vertex-colour alpha

[Tim Day]

I'm writing a script which generates meshes with per vertex colours.
No problems there, but when I vary the _alpha_ component of the colours
blender doesn't seem to render any sort of transparency.
I don't know if this is because I'm not setting the right material
flags, or the blender renderer doesn't support it (or needs me to set
some particular render mode), or whether per-vertex-colour really is
just colour and not alpha.  The attached script alpha.py shows the
problem; if I load it in blender and hit F12 I don't see any evidence of
being able to see through the top triangle to the bottom one.
What's the trick ?

I'm using blender 2.36-1 (Debian Sarge).

Thanks for any help
Tim

[This will be used by a fracplanet save-for-blender option; 
http://www.bottlenose.demon.co.uk/share/fracplanet/gallery.htm 
the CVS head version now generates clouds, hence the need for alpha.]

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