(Aggressive) Caching did it's job. I can't really go into details what kind of hardware we have but that first burst of traffic did not cause any problems at all.

Actually, what you're looking at here is not just a php+mysql+mod_rewrite. You're looking at a real static html file. There is a Sharenator_Dedicated_Servers directory in the server which actually contains index.html file.
This page may look dynamic (so many variables, ajax, etc.) but it's not, we faked it to look like that. I guess I should write an article about this :)

While it may sound a little bit complicated, it saves us a lot of resources. Web servers works much faster handling static files.