Wouldn’t it be quite handy that if someone on another site linked to your blog post, and had some of it quoted and anyone that follows the link, lands on your site and sees the quote highlighted? Maybe the plugin is out there already? I know Wordpress has the Google search keyword referral plugin, but that only looks for keywords in the URI the visitor came from.
I suppose what is needed is a plugin that does a text comparison of the URI the visitor came from and the one on the current webpage. After the first visitor it can tell what was quoted and adjust itself for every new visitor from the linking URI.