Google was pretty prominent when 9/11 happened. Most major news sites went down. Google actually mirrored a copy of cnn.com and linked it on the front page of Google.com under the search box. As far as I remember, Google itself never went down.
I remember the major American news sites like cnn.com and msnbc.com becoming more or less swamped and unreachable by the late morning of 9/11. They came back a little while later with static, text only versions of their home pages for the rest of the day.
That is a very good point to remember when designing high traffic web applications. Add the functionality to switch the site to just text mode on the occasion of overwhelming traffic like the ones described above.
Stop serving images? Obviously creating a text-only template is best, like another poster said, but the images are going to be the biggest drain. Simplifying your layout could help if you are under huge strain, but not as much as removing images.
At one point the CNN page had only a photograph of a person covered in dust and the words "God Help Us." I believe that they had a significant downtown office presence like many other news organizations, so their site crashes could have been more related to that than traffic overload.
It would need to be pretty darn big. 9/11 came the closest, but everything's a lot more robust now. Obama's victory tracked much higher in absolute numbers (~8.5 million pageviews per minute if I recall correctly), and the internet held up pretty well.
