Out of curiosity, how long have you known each other (perhaps defined by when you first received an email from him via your HN submission and when you considered him a co-founder)?
I'd love to know this as well. Also, was their any specific criteria besides technical and that you got along with him when evaluating him as a co-founder? Are you located near each other (i.e. same city) or working remotely? Very cool story, thanks for sharing!