Extending .gitignore

Currently .gitignore serves (at least) 2 purposes:
1. Specifies which files to ignore during git add
2. Specifies which files to ignore during git cleanup, but still deletes them with git cleanup -x
So it effectively splits files in 2 categories.

I always find myself with 3 categories of files:
1. Important files that I want tracked by SCM (normal files like *.c)
2. Unimportant files that I want ignored by SCM and cleaned (usually build files like *.obj, *.exe)
3. Important files that I don't want to be tracked by SCM but also I don't want them to be cleaned either (these are usually 
machine-specific config files)

So I want to be able to say to git: don't track this file, but don't delete it either (even with clean -x).
What do you think? Does it make sense? Can it be done right now?

- Dmitry