I've been waiting for Google to build this into their blogging service for some time. And it's finally here. It is now possible for you to be notified of any follow-up comments in any one of my posts. All you have to do is tick the box ("Email follow-up..") as shown. Just one catch! You'll need to sign up for a Google account. This stops anyone from using someone else's email address.
Hopefully, some of our regular commenters will find this new feature useful. It should help you keep a track of new comments to old posts.