Interesting post on TechCrunch about a new product enabling tracking of Gmail readership (think AOL, circa 1998). According to the article, EmailOracle “inserts an image into each email you send. When that email is opened by the recipient, a call is made to EmailOracle’s servers to get that image. And this lets them know the email has been opened. They then send that information to you by way of a dashboard that is built into Gmail thanks to the plug-in.”
Of course, Gmail (and many other mail clients) allow blocking of images. So the savvy user might well be protected. That said, one might be less skeptical of images from friends, colleagues, or trusted advertisers.
In short, not a huge issue, but still a little sneaky!