This isn't the first time this has come up. I even contributed Working Node Tracker?
, but it didn't quite fit the bill. I was about 80% of the way done with a new version when life took over and it took a back seat. I have never gotten back around to it. The biggest problem is being able to tell when the content of a pre-existing node changes without being too much of a resource hog. There is no way to know, to my knowledge, if a node has changed without doing some sort of a checksum.
If you just want to know when there are new posts to a thread, demerphq is right - use Recently Active Threads.