dlvr.it has just added support for relative item URLs in feeds.  Relative URLs are those without a hostname present in the URL, for example "/article.php?id=1234". 

The situation that we see this arise in most is with Facebook Page feeds since Facebook insists on using relative URLs in all of their feeds.

There is no 100% accurate way for us to determine what the actual URL of the content is, however the majority of users will find our current method satisfactory.  We make an assumption that whatever host the feed was retrieved from should be prepended to any relative URLs (e.g. "http://www.example.com/feed.xml" -> "http://www.example.com" + "/article.php?id=1234" from the example above), however this can be incorrect.

Since some content is gathered from other sources and aggregated together into a single feed, in those cases the URLs will come from many different hosts and prepending the feed's URL host may send users to the wrong location.

There is an official method for making this determination, however it is very rarely implemented by publishers because it requires users to add extra attributes to their feeds.