RAID BSD and ROOT

[peb_(_dot_)_nowarra_(_at_)_t-online_(_dot_)_de]

Dear list,

my first posting here, if not adequate, please re-direct me - thanx

I have tried to set up a RAID like described on the several Webpages, 
amongst those (and "man raidctl" - of course !): 

http://www.abstrakt.ch/unix/MINI-HOWTO-openbsd-raid.html
http://www.monkey.org/openbsd/archive/misc/0203/msg00803.html
http://www.abstrakt.ch/wiki/bin/view/HOWTOs/OpenBsdSoftwareRAID

The RAID works fine, now I want a "rooted RAID". 
So basically I used a second computer to build my kernel with RAID 
support, and copied it to the destination engine, after having installed a 
minimum set of OpenBSD 3.6 on sd0a. If I boot the kernel, I can see with 
dmesg, that it has the RAID support. I then configured my Raid to have 

raidctl -A root raid0

so that a root should be recognized. When I reboot my machine, at the boot 
prompt I tell:

boot sd0a:/bsd -a

it would boot my sd0a kernel, up to the point where it asks, were to find 
root. The options I can see are only floppy, sd0a and sd1a, but no RAID. 
The question comes down to:

How can I "tell" him, to use root from my RAID ?
Have I forgotten something ?

Volker