Ask HN: Unshortening links

I've seen a ton of articles in recent months about URL shorten services (and the dangers associated with using them). I'm curious to know why HN'ers aren't simply "un-shortening" URL's on any sites they control. If you are parsing URL's in your site's code and you maintain a reasonably definitive list of shortening services, why not write a little script that follows the shortened link to see where it really points and replace the shortened URL with the real link?

Including such a feature in your website would also go a small way towards preventing users from posting shortened links (assumed innocent) to known malware-infested sites, if you run something like a blog that allows comments.