Apparently this server is configured to check 'If-Modified-Since' header. You need to pass last modified time as well:
>>> d = feedparser.parse('http://www.wired.com/wiredscience/feed/')
>>> feedparser.parse('http://www.wired.com/wiredscience/feed/',
etag=d.etag, modified=d.modified).status
304
>>> feedparser.parse('http://www.wired.com/wiredscience/feed/',
etag=d.etag).status
200