Ahh... I knew Git was basically started with that Bitkeeper war, but since I don't really follow linux politics much as I don't use it all that often, I didn't realize that hg was started then as well. They seem very similar in their paradigm shift away from the central repository style of CVS and SVN. Is that a coincidence or was Bitkeeper also in this everything is a branch method?
I think it was Bitkeeper's solid decentralized workflow that convinced Linus that VC would even be feasible for kernel development; hg and git were written after people realized that they could no longer do without the features that Bitkeeper had.
Git and hg were both started in April, 2005, in response to the Bitkeeper fiasco.