this feature is already there
there are two ways of doing it
at the top of the thread youll see the header 'Thread Tools'. Youll see an option under this menu which says 'Subsribe to this thread'
All you have to do is select that to get email updates
alternatively when your replying in advanced mode (ie. your not using the quick reply method), under Additional Options (which is below the submit button) youll see an option called 'Thread subscription' - change this to 'Instant Email Notification' to get your updates
hope this helps
