From OpenBSD 4.4 onwards, we have WPA.
For wpa keys, use wpa-psk:
$ wpa-psk unwiredbsd mylongpassphase 0x7579c38e59faaa3b64bd8372e94f74fe7ae2e4e91af154c956a9bfd0240ac9d0
A WPA access point config:
up media autoselect mediaopt hostap mode 11b chan 6 nwid unwiredbsd \ wpa wpapsk $(wpa-psk unwiredbsd mylongpassphase)
Also see FreeBSD WPA setup
From OpenBSD 4.9 onwards it's even simpler:
up media autoselect mediaopt hostap mode 11b chan 6 nwid unwiredbsd \ wpakey mylongpassphase