Did a little checking with wget, and whaddya know but I was trying to get the content from the wrong server. Long story short, there was an /etc/hosts entry mapping the domain in question to the external IP of the 'not working' box - delete that and all is well.
Excuse: box in question is being worked on as a replacement for the existing site, so it will eventually really BE the correct domain (just not yet).
@Joe - thanks for the sanity check, seeing that it was working elsewhere led to the fix!
Word to the (un)wise: check your network stuff!
Feedparser, I'm sorry I doubted you!