Funny thing is, I was just thinking about such a feature a couple days ago. It's pretty easy to send a business email and forget to attach a file that was requested.
However, 5 seconds is ridiculous. It isn't going to be enough time for someone to click the Undo button out of remorse. But most of all, if anyone has any knowledge of the internet, they'll know that 5 seconds sometimes isn't enough time to even load a webpage during peak bandwidth hours.
As an example. Here at work, with a T-1 line, I generally click on a Tomshardware link, then alt-tab to something else to wait for the 30 seconds it takes to download all the Ads on Tom's before the article I actually wanted to see appears.
So, if I had to wait 30 seconds for the page to load in the first place, I'd miss the 5 second rule for the Undo button. 😉