You want the --keep-local command-line option. This removes the file from version control without removing it from your filesystem.
$ svn rm --keep-local my_important_file
Note: The --keep-local only affects the svn rm of your copy. Other users may have their own local copy of the file deleted unless there is a conflict between their local copy and the repository due to changes they have made. This may not be the desired outcome. See comments below.