Google has one internally, and Facebook has done a ton of work on Mercurial to build something similar, doing things like replacing files on disk with memcache servers running close to the dev machines. But I don't think there's anything that works "out of the box" at that scale.
For quite a while at Google there have been git extensions/tools for working with Piper. They're pretty widely used, but haven't been "officially" supported.
There's a new project that allows you to use hg with Piper/CitC; I've been using it for about a month and it's been working really well. No clue how it works, though.
