YaifO 0.2 has been released. This extremely cool tool allows you to remotely install/upgrade to OpenBSD.
I used it for the first time today to upgrade a file server at home. It was running OpenBSD 3.7. On my desktop (running OpenBSD 3.8 with the source tree in /usr/src). I downloaded and extracted YaifO to my desktop and changed into its directory. Copied my authorized_keys file there and edited the config file in that directory. Then, I ran "make obj" and "sudo make". The only modification it made to my desktop system was to install the crunchgen utility that's used to make boot floppy images. Then, it builds a setup boot disk called yaifo.fs and a ramdisk image called yaifo.rd.
Next, I backed up relevant directories on my file server (/etc, /var ) and copied yaifo.rd to it as /bsd and rebooted. When my file server came back up, I could ssh to it as root (with no password - my authorized_keys file) and was prompted with the familiar "(I)nstall, (U)pgrade or (S)hell?" prompt. The rest was a snap. Really. You should set up a test system and attempt an upgrade.
 If you think I forgot to back up some other things, my data is stored in /home and /home is a separate partition. I just didn't have the setup program format or mount it. I did that after the install process.