Having invested so much into Git-oriented projects and whatnot, I think they will remain Git-only till the heat death of the universe.

And then again, Hg repositories on _Git_ Hub?

They've supported Subversion for a few years now: https://help.github.com/articles/support-for-subversion-clie...

They (kinda) support subversion already.

https://help.github.com/articles/support-for-subversion-clie...

I think it's incredibly unlikely Github will ever support another SCM, unless a new one becomes available which all the devs switch to.