This happens automatically when I bookmark a link so long as I add a certain label to it.
Del.icio.us is simply a bookmarking site that allows me to save bookmarks and attach labels. All links that I want to appear on Twitter get the label “aop”. (You can see all my aop links here.) The most important del.icio.us feature from this point of view is that you can retrieve these bookmark lists in RSS format (e.g., this RSS feed for my aop links).
Twitterfeed posts RSS feeds to Twitter, so all you need to do is to tell it the address of your del.icio.us RSS feed and determine the posting frequency etc.
Del.icio.us and Twitterfeed work really well together, and it’s my impression that many of my Twitter followers really appreciate the links I’m posting.