Introducing HTTP Nowhere.
HTTP Nowhere is a Firefox browser extension I recently wrote to implement this idea. You can install it from the Firefox Add-Ons site, and check out the source at github. Beyond what is discussed above, it allows you to see which unencrypted requests were made while in this mode, and lets you add specific URLs or URL patterns to a whitelist.
This is just an initial Firefox-only implementation of the concept. I can imagine a future where the conscious action could be inserting and removing a hardware usb key to get your browser to go into this mode. This could work in concert with existing solutions that store client certificates on hardware devices.