To properly boot my Jornada 620LX I use HPCBoot with:
netbsd.gz
NFS
(NO single-user, but the box above it checked)
Then boot.
It'll prompt for root device (wd0b)
Filesystem [ffs]
Dump location (wd0b)
It'll prompt for init device [/sbin/init - enter]
then it'll boot fine.

It does _not_ boot properly if I tell it to boot directly off wd0b