The most recent thread on the subject started with: RFC: Subscription to nodes. The eventual response by tye was: Re^2: RFC: Subscription to nodes (really?). And an eventual possible workaround came from Limbic~Region with Working Node Tracker?. In the end, Limbic~Region's solution doesn't do exactly what you are asking, but it could be helpful nevertheless, as it provides an offsite tool for assisting you in tracking nodes and threads.
But your question originally was, is there a way to subscribe to threads and be notified of their activity. The answer became a little convoluted, you asked for clarification, and to make it perfectly clear I presented an unconvoluted and factual answer: Re: Re: Re: Email notifications again.
At any rate, check out those links I provided here. They really do tell the story better than I can.
"If I had my life to do over again, I'd be a plumber." -- Albert Einstein