I had a bookmarklet that did that for a bit, and the old LJaddons Fx plugin was good for it, but I've lost both over the years. I would also like such a thing, either inbuilt or as a bookmarklet.

Given we already have the quote button for comments, I'd imagine that similar functionality could be built into a share button and I'd definitely appreciate it. You can already force an entry page to have a modicum of text into the main box, but I never finished working out the details on how to get it working fully as it required putting everything as escaped text into the URL bar which wasn't ideal.