Google Blogoscoped runs this post speculating about an “undo” option for Gmail. I’ve touched this topic some time ago in my “You can’t recall an email” post. The base for that post of mine was purely technical. What is sent is sent, and there is no way to get it back.
With another look on this issue, I see that technical side can be controlled to a certain degree. Webmail providers (such as Gmail, Yahoo Mail, Hotmail, etc), can indeed delay the outgoing message by a few seconds. Countdowns and disappearing buttons aren’t an issue either – we have plenty of technology these days (AJAX, Flash) to implement them. And there is a certain demand for the functionality too – this can be judged by all those browser plugins and extensions, like the one mentioned in the Blogoscoped article.
Still, I’m standing on the side of “don’t do it”. I think it’ll add to the confusion of the interface and the complexity of the system, without too much benefits in return. I don’t think that we should have an “undo” for everything either.  And I think that the old way of “sorry, forgot to attach this document” works pretty well and sometimes makes people to actually read through and think over again about what is that they are planning to send out.
What do you think? Would you like to see an “undo sending” button in your email client?