OpenBSD has maintained their own hardened fork of Apache 1.x for years. I think that switching to 2.x would be a monumental effort for them and set them back quite a ways in terms of security. I think that nginx is a great move for OpenBSD.
Last I knew, it was less about the effort, more about the license. And if you really need/want Apache 2, it's available in the ports tree and as a package. #> pkg_add apache-httpd
