I love the AJAX thread title edit on forumdisplay, and I'd love for it to be added to showthread as well.
I have already added it on my forums-- one plugin, no template edits, it reuses all the same code already used on forumdisplay, and allows you to double-click on the title of the first post to change the thread title. Now can it be default?
Note: what I've done lets you change the title from the first post. I'd prefer the thread title to be the heading for the page (above the list of posts) and have it editable from there, regardless of which page of the thread you're on. That would be a good improvement for the 4.0 style.