Reply to messages via mail

It would be great to be able to reply to messages via mail, maybe making a mailing address with a hash or something so that the reply would be sent to that threat. Or, as I've seen with Google Docs Spreadsheet Forms, embed a form into the message...

This would be very convenient. How difficult would it be to implement this?

This is an often discussed feature. We want to do it, but its not easy to do and is usually quite dependent on the specific server configuration of the installation.