For someone aiming to protect the entire Internet, maybe there is interest, but you’d have to get highly heterogeneous to even make it work. (The largest worm I am aware of maybe compromised a few hundred thousand machines – even limiting to a million machines per O.S. or application type would require 600 different heterogeneous solutions).

I found the argument about heterogenous environments far more convincing than the ‘secure because not Microsoft’ assertion. Of course that argument never asserts that you will have a more secure syste, just one that is less likely to suffer from the same attack that targets a different system.