Hi,
check out this one:
https://github.com/urso/dotrc/blob/master/vim/syntax/haskell.vim
A (not up to date) version of it can also be found on vim.org:
http://www.vim.org/scripts/script.php?script_id=3034
It is not an official version, but already a modification of the original syntax file, plus some new features.
Unfortunately I haven't had time to maintain and update the syntax file for a (loooong) while, but hopefully next week I will have some time to do some maintenance. I plan to incorporate out standing pull requests, do some improvements and split of the script itself into a new project on github with test cases, readme and screen shots.
If you've done any changes which may benefit the syntax file I would be glad about patches or pull requests on github.
I'm using github at the moment, but am open for suggestions about other hosting ideas (e.g. bitbucket plus github mirror or hackage). As I've made some extensive changes I will continue maintaining the syntax file (unless someone else really wants to do it...), but I'd prefer it to be a haskell-comunity project so other people can join in easily and propose changes.
- Steffen