This is a request for a feature I like to have on my most used forums.
Basically, on any topic, you should be able to choose whether to subscribe to the post and receive e-mails when any new replies have been posted, including the reply itself.
If this is possible it would be a great addition as when I'm out of the house, I check my e-mail using my mobile phone, and would love to see the updates in there.
Failing this, or in addition to, if it is even possible on SMF, maybe we could have SMS subscription, where updates are sent via SMS (including Private messages). Very similar to what they have on Facebook and Twitter, and would also include a reply option (simply reply to the SMS with what you want to post and it automatically posts for you).
I don't know if anybody else would like these features but it would certainly be a useful addition to me at any rate.