iTunes 4.9 supports PDF enclosures!
/By accident I discovered that iTunes 4.9 downloads PDF enclosures from a podcast feed:
It even has a document icon next to it. If you double-click it, it opens in Preview or whatever PDF viewer you've configured.
(Movie enclosures are of course also supported and have a movie icon.)
I wonder what other enclosure types iTunes recognizes...
(OPML maybe, for shownotes or multiple enclosure support?)
2005-06-30, Update: Here are some details.