We are currently extending the forum with this functionality. In MXDN you can configure this on the My Account page.
The forum has been updated. You can now configure email notifications for each question.
See the section "Subscribe to receive email notifications" below. You can define the frequency of email updates. Email updates will be send if a comment or answer has been added, but only once in 15 minutes, once a day, or once a week (digest email).
On the profile page an additional tab has been added "Email notifications", which enables you to manage all your subscriptions.