I wanted to do the same thing a couple of days ago, so I put together this simple script: https://code.google.com/p/getgrfeed/
The script can be used to download the entirety of a single feed stored in Google Reader. For instance, if I wanted to dump the feed for BoingBoing, I'd run it like this:
$ python getgrfeed.py http://feeds.boingboing.net/boingboing/iBag
It will then start printing every entry Google Reader has stored. Of course, BoingBoing is a busy site, so it would take a while, but it should work. This is useful especially to save a copy of blog entries -- or entire sites -- that no longer exist anywhere else.