It's important to remember the destructive nature of Truecrypt's hidden volume function. Unless you use both keys to unlock the volume, opening the outer volume and allowing the OS to load will destroy the hidden volume that has your real files. It's meant to be a last-resort against a "Use this wrench and beat the password out of him" scenario.
I just ran the same test and can confirm the results. Google will only load your image if you open the email, which means Google has just opted-in all users to mail receipts.
I don't use any Google services outside of small tests like this, but it still makes me concerned for how this will affect the privacy of people I know.
Thanks to this post, I decided enough and enough and I started on a user script that redirects different URLs to their "pretty" version. It currently supports Google Web Search (Google Instant Search is not yet supported), although I'll be adding much more when I get home and in the next few days. I've named the script "Prettify-URL" and it is available here:
Note that this is absolutely not meant to be an end-all solution to the problem, but instead a ray of sun in a thunderstorm of ugly URLs. The core responsibility still lies on the developer, this just tries to make things a bit more bearable.
I haven't seen web servers send back different pages for non-Mozilla browsers since Mozilla browsers were selectively served frames[0], which is why most User-Agent strings contain "Mozilla". Despite whether this is a good idea or not, it's nostalgically entertaining to see history repeat itself like this.
I'm all for Google fixing their mess of communications platforms, but by attempting to consolidate those into a proprietary solution, I fear they've doomed it to the same fate as Google Wave.
If they were to at least include support for XMPP clients, they would be golden; however, the opposite appears to be true.
Or, if they snip the GET variable for whatever reason (I don't see them doing this):
http://example.com/gmail/{yourusername}/trackingimage.php
Or even:
http://example.com/{emailidfromadatabase}/trackingimage.php
This tactic is already in use by most mass email companies.