I've noticed that Particls seems to have a problem dealing with ATOM feeds that use relative URIs. When I click on a link in the sidebar (may be other places as well) of an entry that uses a relative URI, I get sent to a different blog from my OPML with that link tacked on.

For an example, add Sam Ruby's blog and see what I mean: http://intertwingly.net/blog/index.atom 

Unfortunately we don't support relative URLs as you have discovered - but good catch.  I'll chuck it on the "todo" list and get to it ASAP.