>> `dired-pop-to-buffer' could bind `split-window-preferred-function' to a
  >> function that tries to always split the selected window instead of
  >> whatever window was chosen by `display-buffer' but that would override
  >> any user customizations of `split-window-preferred-function'.
  >
  > Why would it override it?  It would of course bind
  > split-window-preferred-function to a function that selects the right
  > window and then calls the previous value.

As a matter of fact, we do have to override the users' customizations
here just as Juri suggested earlier.  Suppose a user has set
`split-window-preferred-function' to the standard option "horizontally"
which always splits a window horizontally.  Then `dired-pop-to-buffer'
simply cannot get a vertical split when it calls the original
`split-window-preferred-function'.  So we probably have to do this as in
the attached patch :-(

martin