Your comments

Ha, I'd actually forgotten about Command+D, so thanks for that. :)

Yep, Command+D works, but is obviously more tedious, and will occasionally highlight text I don't want (substrings in other words that are unrelated). It also doesn't respect scope, but I guess I can use it regardless.

ybakos, you're right. That's probably the best reason for not implementing this.
No problem, glad I could help. :)
Daniele, you make good points. Might I point you to this discussion, then?
You might have better luck posting your mockups there (it's a highly discussed and upvoted request)... :)
From a programming standpoint, this isn't a trivial thing to do. And it doesn't seem to make much sense from a usability standpoint in my opinion either (remember those top tabs in the Safari 4 beta?)... What happens when you drag the tabs? Or when you have so many tabs open the whole top bar is covered in tabs — how do you move the window?