Thanks to http://www.google.com/support/forum/p/Chrome/thread?tid=70b1d11d50817e04&hl=en
1. Right-click anywhere on your Gmail page and click "Inspect Element".
2. A debug window will take over the bottom half of your browser. Click "Console" at the bar at the top of this window.
3. Search for the words "insecure content" in the console (search bar at the top-right of the console). This line will reveal where the insecure content is coming from.
4. Disable the plug-in that is creating this message. In my case it was Zemanta, which neither one of us realized was even a gmail plug-in.
