Just uploaded a small WordPress plugin, URL Absolutifier, that converts relative links and image sources in WordPress RSS/Atom feeds into absolute URL. For example, from
<a href="/code/url-absolutifier">URL Absolutifier</a>
<a href="http://scott.yang.id.au/code/url-absolutifier">URL Absolutifier</a>
It was written because (1) I like to use relative URL when I write my blog posts (2) feed should not contain relative URL references (3) some feed readers can't handle them, most notably Bloglines. I do not agree that it's feed readers' fault, as feed validator isn't happy about it either. Fortunately WordPress provides sufficient hooks for 3rd party developers to fix up its brokenness.
Grab yours here.