How to “unversion” a file in either svn and/or git

Question

It happens to me all the time. I accidentally version a file, I do not want to be versioned (i.e. developer/machine specific config-files).

If I commit this file, I

Answer 1

I there f I choose to never commit the file, I always have a "dirty" checkout - I am unhappy.

With regard to this particular point, you might want to .gitignore the file as other have suggested, or to use a scheme like the one described in this answer of mine.