Haven't tested it out, but the latest NetNewsWire beta now supports downloading enclosures (even automagically into iTunes). It didn't trigger on all of the old enclosures in my 300-odd feeds (I shudder to think about all of the bandwidth sucked up if I have to download all enclosures in all feeds...), and there haven't been any new ones today to test, but this looks promising!
There are a whole buncha updates in the latest beta, too. Feels much snappier (but that might be an optical delusion), and the UI has been cleaned up a bit (including a CSS Zen Garden-ish style sheet selector widget).
Smart List options still don't include "Date" (for selecting items within a date range, for instance), but I'm sure that's in the works...
UPDATE: My personal pet peeve bug in previous betas appears to have been fixed. In older versions, if I was reading items that were trickling into the New Items folder as the feeds were being updated, all kinds of funkiness could happen, often with things like "Grr!" or "Crap!" accompanying them...
UPDATE: Podcast download worked like a charm - it just automagically picked up Dave Winer's New Year's Eve episode of Morning Coffee Notes. Neatly added to iTunes, with proper genre etc... Cool.