For now just a todo list for a proposed new 'splitvertical' feature.
Initial patch by Christian Brabandt: http://groups.google.com/group/vim_dev/msg/7340b8005d1740d4
0. Write documentation for splitvertical, update interacting settings as we find them.
1. Update syntax files for [no|inv]spv and [no|inv]splitvertical
2. Add a :horizontal command
3. Trumps :set diffopt+='horizontal' right now; is this the right decision?