Thank you! To give credit where it's due, buro9 from HN urged us to implement the Varnish caching feature for cached pages, so we had it written but disabled because we didn't think it was going to get that much traffic. We hadn't turned it on for the first 30k visitors and the server was still doing great, but when we saw it we just switched it on and load dropped to nothing.

Serious amounts of love for Varnish here.