I have a working beta version of the Real Costs script up and running. This Greasemonkey script pulls each flight origination and destination information from the page, calculates the CO2 produced, and reinserts that information next to the flight. It also inserts a graphical comparison of the CO2 produced for that flight in comparison to making that trip by bus or train, and to other metrics such as the average CO2 produced per capita for the average US citizen and the average world citizen.
It looks good, and works well on the bulk of the major US air carriers. For some reason, most of the international air carrier websites break the script, no matter what I do (there are a lot of scary iframes out there...) A list of these carriers and documentation of all scientific calculations is available on the project Wiki.
Let it be known that this is a beta script. Which means, expect that it will break at times, and please tell me about it! I will be working on the website during the coming week to get installation info and better documentation up. I will also compile an .xpi for easier installation. At that point I will do a bit more announcing.