If the user has chosen to receive emails of discussion posts, then when someone edits an existing post, they will automatically get emailed the "change".  However, when I just turned off the "sticky" option and didn't change the content/text of the discussion, it still sent an email by default.  Even if changes to the text are made, it would be great to have a checkbox that is automatically set to "on" that says "Resend email?".  This can be turned off by the editor to reduce spam.

Note:  Just saw that this was an option when editing an Announcement in Groups.