how is it done?
1) The email isn't actually sent when you click send. There's a delay where you can go back and edit/delete it. This is effectively the same as saving an email as a draft and then having it auto-send five minutes later.
2) The email is actually just text on an external image or on some external web page visible through an iFrame. Neither of these really work because modern day email services like Gmail cache images and pass them through their own servers. Most email clients also block iFrames by default.
Based on their website I assume UnSend.it uses one of number 2. An interesting hack and definitely Hacker News worthy, but in practice I don't think it can be reliable.
There is also always the possibility that I'm wrong and there's a secret method #3, but I really doubt that.
Anyway, nice idea!
Edit: I signed up and you were right. He uses a remote image. It works when I email using the unsend.it web app but doesn't work if I send it through the Gmail website so I couldn't test the attachment.