Vim and TeX keywords

After updating my vim installation, my command set iskeyword+=: in ~/.vim/after/ftplugin/tex.vim stopped functioning. (This was recommended by the Vim-LaTeX project.) My auto-completion for chapter references chap:intro and equations eq:blahBlah was broken because the colon was no longer interpreted as a command character.

To make a long story short, I found the :verbose set isk command which told me the last file to set iskeyword:

/Applications/MacVim.app/Contents/Resources/vim/runtime/syntax/tex.vim

Somehow, that syntax file is hard-wired to reset iskeyword. The solution is to add the following line or equivalent:

let g:tex_isk='48-57,a-z,A-Z,192-255,:'