Do you really need a plugin for this?

If you want relative numbers to appear when in normal mode, and absolute line numbers to appear otherwise (e.g., when the window has lost focus), just throw this in your .vimrc:

    set rnu
    au InsertEnter * :set nu
    au InsertLeave * :set rnu
    au FocusLost * :set nu
    au FocusGained * :set rnu

Here's a gist for posterity. Feel free to fork it, leave comments on what should be changed, etc. The above has worked for me on Mac OS X for years, but there's a chance I didn't set it up the "right way". Leave comments on this gist and I'll update as necessary:

https://gist.github.com/3012145