Mike,
read receipts have to be turned on in the email client for this to work.
Since no-one in their right mind ever has this turned on (for spam reasons) adding it to an email is basically useless.
tracking that an email is read is hardish depending on how slimy you want to be.
Kevin's approach of an embedded link - such as to an image, is likely the best, although in many readers the user has to "ok" the images before they are fetched.
cheers
Bruce