Are You Pondering What I'm Pondering?

Why hasn’t anyone implemented an automatic trackback system? It would be like a referrers page, but in addition to listing the site someone came from, it would spider the site and create a snippet of the text around the link. Certainly there are problems with linking to the front page of a site rather than a permalink or with spam (there’re always problems with spam), but it doesn’t seem insurmountable.