The sad part is that I end up having multiple requirements files anyway to match the different environments (test, production, development) -- see the django applications for this. IMHO (again) we'd need some sort of structured requirements for different environments.

If I install an app/package for development, then I'd need development dependencies (like test-related packages etc.). I'm not supposed to know/care what the third party code needs for testing; just when I start testing the relevant code should "just work". Also, if I'm in production the testing-related packages should not pollute my environment...