This:

  > Screen has had vertical splitting for
  > sometime now, just get a beta release
  > or build from source

and this:

  > Screen is everywhere. I can go from
  > system to system and it'll be there.

Seem to be opposing ideas. Seems to me like 'vertical screen splitting' isn't available everywhere. It's much more likely that old builds of tmux have the function, while old builds of GNU screen don't.