We installed a third webhead right before the debate. About 30 minutes into the debate, we knew that three wasn't enough, and we threw a fifth machine on our cluster.
This may be boring stuff, but I tell the story to impress just what a killer job the dKos tech team did. Jeremy has been a hero the past two weeks busting his ass to keep up with the site's ridiculous growth. Meanwhile, the folks who host my servers -- Voxel -- were incredibly responsive, and have worked tirelessly the last couple of weeks to make sure we could handle the growth. I couldn't have done any of this without their help.
Update: Oliver Willis has the video of Bush totally losing it.
Update II: LOL, from the comments: "I need some wood for my Internets".
Update III: For the geeks, here's a chart of my bandwidth. For comparison purposes, Slashdot does 120 mbits per day (though their site is quite a bit heavier than the relatively svelte dKos).
This is the one-week view. Tuesday's traffic had been the site's previous high (the veep debate). So today, traffic was over twice my previous high.