John Gruber:

I.e., three blank lines followed by “Read more:”, then the URL from which the text was copied, then an identifying hash code used for tracking purposes.

and:

Everyone knows how copy and paste works. You select text. You copy. When you paste, what you get is exactly what you selected. The core product of the “copy/paste company” is a service that breaks copy and paste.

If you have ever copy and pasted text from a web page and there was an auto inserted line that said “read more” with a URL this is what Gruber is referring to. He offers a great solution, this was a huge annoyance for me when I am creating all these link posts. Thanks Gruber!