I'm not sure people will actually want this change, but I like it, so
I'm providing a patch. All this does is change org-agenda-next-line and
org-agenda-previous-line so that they move by agenda items, and not by
actual lines.

The only real drawback I can see is that, since
`substitute-key-definition' is being used to rewire *all*
next/previous-line definitions, it becomes a little difficult to
actually move by single lines. But if that's not a big deal…

Eric