Working on it. The problem is that the new package symlinks libncurses.so.5
to libncurses.so.4 (they're 100% compatible), then the old package gets removed
and removes the links because they're identical with files that used to be in
the old package.
A workaround is to do
rpm -U ncurses-5.0-2.src.rpm
rpm -U --force ncurses-5.0-2.src.rpm
in a row.

Surely RPM is intelligent enough to know not to delete files in the old
installation that are replaced by files from the new installation?

In pseudocode, its logic should be something like this:

 1. Create list of files in the old installation. Mark these all as
    "To be deleted".

 2. For each file to be installed or updated by the new installation
        If this file is already in the list of files then
            Mark it as "To be updated"
        else
            Add it to the list of files, marked "To be installed".
        End If
    End For

 3. For each file in the list
        Case "To be flag" of
            "Installed" : Install the file.
            "Updated"   : Replace the file.
            "Deleted"   : Do nothing.
        End Case
    End For

 4. For each file in the list
        If the file is marked "To be deleted"
            Delete the file.
        End if
    End For

 5. End of this procedure.

Note that step (2) includes files created by any post-install scripts.

It seems that the "*.so.5" files are now completely missing in the version
"ncurses-5.0-3.i386.rpm"
That can't be your solution because the symbolic links in the package link to
"/usr/lib/*.so.5", but as I mentioned those files are not there!...

bero, any update on this?  you may have to use triggers.  Talk to jbj about it.

It's been fixed by renaming the .so.5 packages back to .so.4 a while ago; sorry,
forgot to put it in bugzilla. The additional plus is that packages compiled with
ncurses >= 5.0-5 will again work with systems using ncurses 4.x