The version available via git from savannah.gnu.org has the old vertical split patch plus a number of more recent scrolling performance fixes. Thanks for your list of non-ideological reasons, better notifications sound pretty compelling. I've been meaning to hack growl support into the OSX screen for a while now...

It's not in the version of screen that ships with OSX. The macports version of screen supposedly supports vertical splits with the +vertical_split variant, but that didn't work for me last time I tried. And that's how I discovered tmux.