That sounds like a good idea to me. I find it damn near impossible to tell when a comment thread has ended, because the only indicator of a new top-level comment is the number of blank pixels to the left of it!
I would _really_ love a collapse button that floats down the screen as I scroll, so I can hide an entire reply chain after I'm deep into it already.
That's a good idea but how would you recall which one you are collapsing? Color code them? Hover over the button to show the original comment as a tooltip?