Since it came up here recently and opinions wether it would work
diverged I tested booting from a raid 1 set configured as auto root.

It did not work. But it was easy to fix, despite me not speaking forth.
The fix got pulled up in the meantime, so both -current and 2.0 are
able to boot from root on raid 1 without helper partitions.

The way you do it is described in the NetBSD guide

  http://www.netbsd.org/guide/en/chap-rf.html

just ignore anything about MBRs and fdisk ;-)

Basically you end up with two partitions (say wd0a and wd1a), both type
RAID, a raidset configured with them, marked as auto and root. Then
you copy kernel and ofwboot to / (on the raid file system), and then
install bootblocks like

  installboot -v /dev/rwd0a /usr/mdec/bootblk
  installboot -v /dev/rwd1a /usr/mdec/bootblk

Thats it.

Martin