I'll answer my own post. I've come to the conclusion that the OpenBSD IKEv2 implementation in iked is incompatible with Cisco. It works between OpenBSD boxes but none of the several Cisco ASA devices I've tried with did I get it to work. Switching to IKEv1, i.e. ISAKMPd, works immediately.
/ Danial ----- / Danial -- Sent from: http://openbsd-archive.7691.n7.nabble.com/openbsd-user-misc-f3.html