Gmail: Display when Recipient Reads Mail

This is an interesting/useful/privacy invading tool that could change the way we email today. I saw a request for a tool that would tell you when an email you sent was opened by the recipient. This tool could be used to bring about a new age of spamming, where you guess addresses, and if the mail gets opened then you know it's a real address. I don't know if this is even possible with current email standards, but I thought I'd throw it out there.