Well this was just one of their cute little april fool's jokes: http://mail.google.com/mail/help/customtime/index.html
It is easy to alter the timestamp of sending emails, but the problem is that some email programs on the receiving end don't honor that timestamp, and just mark the date received as when they actually receive it. We actually used to have this feature, where you could select any date to send, and if you chose a date in the past, it would be sent immediately with the timestamp in the past. The problem is that this feature had no real (or honest) use, and when people made a mistake with the year or something, the email would be sent immediately when they didn't intend it to be sent for a while, and they didn't have time to correct the mistake. So now only emails to the future are allowed.