This is really just an extension of clickjacking - modifying the UI to trick the user into performing an undesired action. This is a pretty novel idea, and considering how many websites make use of this to slap their permalinks into copied text (albeit with flash, usually), I'm surprised this hasn't been thought of before.

It would be an interesting experiment to sneak a harmless command after every snippet on a site like commandlinefu.com.

Edit: Also while playing around, I remembered irssi actually has a defense against this. If you try pasting multiple lines, it can detect this. It presents you with a prompt asking if you really intended to paste >5 lines into the text field. I wonder if something like this could be implemented in a shell?