Apparently Google has it's own implementation of Mercurial, and it uses the famous BigTable persistence mechanism. This only reinforces the fact the Mountain View company has a strong pythonic culture ;-)

But they certainly did not choose Mercurial because of its python roots only, they published an interesting Analysis of Git and Mercurial. 

The implementation is avaiable for those who request on the Google Code project hosting service.

I'm personally waiting for a great IDE suport for these DVCSs, until that I'll probably focus on other tools/APIs/frameworks and keep using Subversion.