Mark Gibbs explains how DidTheyReadIt's technology works in tracking your sent email.

The technique employed is conceptually simple: Embed an image in HTML mail to be retrieved when the recipient renders the message content and record the image retrieval details. It is a technique loved by spammers who use it to determine whether the address is "hot" - whether a warm body or something that appears to be one is on the other end.
But DidTheyReadIt aims to take the uncertainty out of delivery and remove the excuse, "Sorry, I never got your message."

USA Today also published a story about DidTheyReadIt.

-Chris