I have made a vim syntax highlighting script for LLVM. It can be found at llvm/utils/vim/llvm.vim. You should copy this file to your ~/.vim/syntax/ directory and add the following lines to your .vimrc file to have it automatically load on a .ll file.
au! BufRead,BufNewFile *.ll set filetype=llvm
Misha claims to be making a script for all you emacs fans out there so look for that soon.