The upcoming OpenBSD 5.3 release includes boot support for crypto softraid partitions, which allows for full disk encryption. The installer has all the pieces, but they aren’t automatically put together. Some assembly required. The basic installation procedure has been outlined by Ryan Kavanagh, but here’s the summary. Only the initial configuration is even remotely tricky, after that it just works.
Boot install CD or bsd.rd. Either pick shell or escape to one by pressing ! during installation before getting to the disk setup part. Create new disk devices by running sh MAKEDEV in dev. Run fdisk and disklabel to create a RAID partition on the physical drive. Run bioctl with all the magic options to create the softraid drive. bioctl -c C -l /dev/sd0a -r auto softraid0 Continue the installer and install to sd1. Magic.