Yes to DNS, though we have to be careful here since DNS can be spoofed more easily than github or twitter proofs over https. I was thinking a slightly better way to prove ownership of foo.com would be to post a proof at https://foo.com/_keybase (or something similar). To spoof this, an attacker would have to spoof DNS and also the https certificate.

Authenticating a self-signed domain certificate via keybase is a neat idea, but would probably need some browser support, unless there's a clever hack that I'm not thinking of.