root mount error with 8-CURRENT on MacBook4,1

I am having trouble upgrading the kernel to 8-CURRENT (revision
197444) on my MacBook4,1.
I get stuck in the mountroot> prompt:


>Trying to mount root from ufs:/dev/ad6s3a
>
>ROOT MOUNT ERROR:
>If you have invalid mount options, reboot, and first try the following from
>the loader prompt:
>
>          set vfs.root.mountfrom.options=rw
>
>and the remove invalid mount options from /etc/fstab.
>
>Loader variables:
>vfs.root.mountfrom=ufs:/dev/ad6s3a
>vfs.root.mountform.options=rw
>
>[manual boot prompt]
>mountroot> ?


I have only modified machdep.c according to
http://lists.freebsd.org/pipermail/freebsd-current/2009-August/010778.html
by adding an entry for MacBook4,1 to cpu_startup(). I am able to boots
this same laptop with 7-STABLE.

Can someone please help?

thanks.