The real semantic markup for a Peter's not-really-a-haiku would be
<haiku><first-haiku-line>I think
that</first-haiku-line><second-haiku-line>this
looks</second-haiku-line><third-haiku-line>much
better</third-haiku-line></haiku>
This pleading for hard line breaks is all nonsense, special pleading to
allow just this one oh-so-necessary bit of procedural markup. This
slippery slope leads to Postscript!!!
(Ah, that feels better :-)#
That being said, Peter's suggestion to allow <p poem>, etc., sounds
quite reasonable to me. It should be clear, though, that no special
handling of line breaks within such a markup is promised, but that
browsers that know the meaning of the paragraph modifiers may "do the
right thing".
Bill