An OpenBSD install does most of that currently, but you just have to press "enter" a lot. As for doing the pkg_installs and other customizations, there is (and has been for a long time) a mechanism to do that, although it's harder to track down the documentation if you don't know what you are looking for. The installer will pull a sitexx.tgz or sitexx-hostname.tgz install set if it is on your installer source.
The FAQ has a lot of details here:
The one other big tip I'll give you that is also mentioned there is the rc.firsttime(8) script mechanism. If you drop a file at /etc/rc.firsttime, those commands will be run at the next boot and then the file is deleted. If doing so in the installer, append to that file, since the installer uses it to.
I saw some rumblings on the mailing lists that a developer is working on an even more automated mechanism for the installer too...
UPDATE 2013-10-29 - And here's the beginning of the new automation method, still very rough: http://marc.info/?l=openbsd-cvs&m=138291339126976&w=2