It’s simple for senders to do this. Embed in each message a viewable image—or if you’re feeling sneaky, a nearly invisible image—that contains a long, random-looking string in the URL that’s unique to each receiver or e-mail. When Google proxy servers request the image, the sender knows the user or message corresponding to the unique URL is active or has been viewed. In Moore’s tests, the proxy servers requested the image each subsequent time the Gmail message was opened, at least when he cleared the temporary Internet cache of his browser. That behavior could allow marketers—or possibly lawyers, stalkers, or other senders with questionable motives—to glean details many receivers would prefer to keep to themselves. For instance, a sender could track how often or at what times a Gmail user opened a particular message.

via Dear Gmailer: I know what you read last summer (and last night and today) | Ars Technica.

The key to this issue is that Gmail now defaults to images on in email which should always be off.  In order to fix this Google must cache all images upon receipt of every email.  Doing it when a user requests an email defeats the entire purpose.  It’s always good practice to view with images off on all email no matter what the provider claims.