A single WordPress can push hundreds of millions of pageviews a month; it can serve tens of thousands of concurrent logged-in users; and it can be lightning-fast the whole time. It is known. The question is not whether WordPress itself can scale, but whether or not your implementation is ready.
This site contains best practices from teams with real-world experience in delivering WordPress at scale. They provide a solid foundation for planning a scalable implementation, both for development teams and site owners alike.
None of my WordPress projects are near this scale, yet (ever the optimist), even so, this site has some quality links to resources and implementation guides. Definitely worth a bookmark, for the future.