I had this itch to figure out over the weekend how to save a document with sudo from vim. With help from Tim Pope, I now know how. Here is how:
Relative line numbers in vim from Bryan Liles on Vimeo.
Update For those of you who want to get right to the thick of things, stick the following in your vimrc
command! -bar -nargs=0 SudoW :silent exe "write !sudo tee % >/dev/null"|silent edit!