I've committed the Poem extension to SVN and activated it on Wikisource. Pretty straightforward, works like this:
<poem> bla bla bla bla
bla bla </poem>
The inside text is wikitext, but line breaks and initial spaces are preserved. Output is wrapped in a <div class="poem">, so you can do global styling of poems per-wiki.
Additionallly you can add other allowed HTML attributes on the <poem> tag just as you would for a <div> or <pre>, such as setting one-off custom styles or additional CSS class names.
-- brion vibber (brion @ pobox.com)