Git failing to ignore .DS_Store defined in global ignore

I'm fairly new to git and hope I'm not doing something terribly silly. I've been living without a way of ignoring files in Git and have finally had my fill of accidentally adding OS files to my repositories.

I have a new git repository the git status of which is:

# On branch master
#
# Initial commit
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   .DS_Store
#   README.markdown
nothing added to commit but untracked files present (use "git add" to track)

Running git add . followed by git status produces:

# On branch master
#
# Initial commit
#
# Changes to be committed:
#   (use "git rm --cached <file>..." to unstage)
#
#   new file:   .DS_Store
#   new file:   README.markdown
#

Running git config -l shows:

core.excludesfile=/Users/Chris/.gitignore
core.repositoryformatversion=0
core.filemode=true
core.bare=false
core.logallrefupdates=true
core.ignorecase=true

The contents of /Users/Chris/.gitignore are:

# Compiled source #
###################
*.com
*.class
*.dll
*.exe
*.o
*.so

# Packages #
############
# it's better to unpack these files and commit the raw source
# git has its own built in compression methods
*.7z
*.dmg
*.gz
*.iso
*.jar
*.rar
*.tar
*.zip

# Logs and databases #
######################
*.log
*.sql
*.sqlite

# OS generated files #
######################
.DS_Store?
ehthumbs.db
Icon?
Thumbs.db