I've been thinking about Enclosures with the Screencast, so I decided to pick up a half-finished plugin I started awhile back. If you have the development version of BottomFeeder, check the updates - you'll see EnclosureHandler as a plugin. If you grab that, you'll have a tool that will go through all the enclosures BottomFeeder references and download them - by default, at 2 am. You can change that via the settings - look in the "Plugins" menu for a small manager tool that allows you to either launch a downloaded enclosure, or view the item it came from.
Enjoy, and let me know if you have problems with it. Oh - if you decide you don't like that behavior, simply delete the file "EnclosureHandler.pcl" from the "plugins" directory and restart BottomFeeder.