Old news for most web-savvy surfers, and a wake-up call for the rest. Very, very easy to avoid being bitten by this one.
http://reviews.cnet.com/4520-3000_7-5138076-1.html?tag=cnetfd.sd"The people in my personal focus group (my wife, my mother, and some coworkers at CNET) agree that this is one of the creepiest things they've ever heard of: a new service that will tell your correspondents exactly when you opened the e-mail they sent you. It will also tell them how long you took to read their message and which computer you used to do so. The kicker: You'll never know all this information is being collected. It's a supercharged return receipt that's completely invisible.
The service is called DidTheyReadIt. What it does is insert a small tracking device, often called a Web bug, into the e-mail that you want to track. When your recipient opens your message, the bug (a one-pixel, transparent GIF file) is pulled from the DidTheyReadIt server, generating a logged event that shows when the message was opened and for how long.
DidTheyReadIt adds presence to e-mail; with this live tracking, e-mail becomes similar to instant messaging. With IM, you can tell if your recipient is online and awake; with e-mail, to date, you haven't been able to. DidTheyReadIt changes that. In fact, it goes beyond IM, by hiding the fact that people are watching your activity. Most IM systems at least require that you approve the addition of people to your buddy list before they can see your presence."
http://www.didtheyreadit.com/...and:
"Disable didtheyreadit.com email tracking bugs"
http://www.macosxhints.com/article.php?story=20040523221845391...and for more information on who is reading your e-mail:

"The Singing Senators are Senator Jim Jeffords of Vermont on tenor, Larry Craig of Idaho on lead, John Ashcroft of Missouri on baritone, and Senate Majority Leader Trent Lott of Mississippi singing bass."
http://www.oakridgeboys.com/Pages/senators_branson.html