JavaScript highlighting inside underscore.js templates for vim

.vimrc

" Add this to the bottom of your ~/.vimrc to enable jst highlighting
au  BufNewFile,BufRead *.jst set syntax=jst

jst.vim

" Put this file in ~/.vim/syntax/jst.vim


" Vim syntax file
" Language:	Underscore.js Templates
" Maintainer:	Martin Schuerrer, @MSch <martin@schuerrer.org>
" Version:	2
" Last Change:  2011 Jul 3

" Read the HTML syntax to start with
runtime! syntax/html.vim
unlet b:current_syntax

if exists("b:current_syntax")
  finish
endif
syn region jstBlock containedin=ALL start="<%=" keepend end="%>" contains=@htmlJavaScript,htmlCssStyleComment,htmlScriptTag,@htmlPreproc
syn region jstBlock containedin=ALL start="<%" keepend end="%>" contains=@htmlJavaScript,htmlCssStyleComment,htmlScriptTag,@htmlPreproc


let b:current_syntax = "jst"

The exists conditional check is redundant because you've just unset that variable. I don't think you need lines 12 to 16 at all.