Setting vim filetype with modeline not working as expected

So, after some digging, it transpires that the system vimrc shipped with OSX sets the modelines (note the trailing 's') variable to 0. This variable controls the number of lines in a file which are checked for set commands. Setting modelines to a non-zero value in my .vimrc solved the problem.

Full output, for the curious: the output of vim --version prompted me to check the system vimrc:

% vim --version
VIM - Vi IMproved 7.3 (2010 Aug 15, compiled Jun 24 2011 20:00:09)
Compiled by [email protected]
Normal version without GUI.  Features included (+) or not (-):
...
   system vimrc file: "$VIM/vimrc"
     user vimrc file: "$HOME/.vimrc"
      user exrc file: "$HOME/.exrc"
  fall-back for $VIM: "/usr/share/vim"
Compilation: gcc -c -I. -D_FORTIFY_SOURCE=0 -Iproto -DHAVE_CONFIG_H -arch i386 -arch x86_64 -g -Os -pipe
Linking: gcc -arch i386 -arch x86_64 -o vim -lncurses

Looking at the system vimrc:

% cat /usr/share/vim/vimrc 
" Configuration file for vim
set modelines=0         " CVE-2007-2438
...

Led me to the modelines variable. It appears that MacVim does not source this system file (perhaps looking for a system GVIMRC instead? :help startup isn't clear).

VIM - Vi IMproved 7.3 (2010 Aug 15, compiled Jul 27 2011 19:46:24)
MacOS X (unix) version
Included patches: 1-260
Compiled by XXXXX
Huge version with MacVim GUI.  Features included (+) or not (-):
...
   system vimrc file: "$VIM/vimrc"
     user vimrc file: "$HOME/.vimrc"
      user exrc file: "$HOME/.exrc"
  system gvimrc file: "$VIM/gvimrc"
    user gvimrc file: "$HOME/.gvimrc"
    system menu file: "$VIMRUNTIME/menu.vim"
  fall-back for $VIM: "/Applications/MacVim.app/Contents/Resources/vim"
Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H -DFEAT_GUI_MACVIM -Wall -Wno-unknown-pragmas -p
ipe  -DMACOS_X_UNIX -no-cpp-precomp  -g -O2 -D_FORTIFY_SOURCE=1
Linking: gcc   -L.         -Wl,-syslibroot,/Developer/SDKs/MacOSX10.6.sdk -L/usr/local/lib -o V
im -framework Cocoa -framework Carbon      -lncurses  -liconv -framework Cocoa    -fstack-prote
ctor -L/usr/local/lib  -L/System/Library/Perl/5.10/darwin-thread-multi-2level/CORE -lperl -lm -
lutil -lc -framework Python   -framework Ruby