zroot won't mount after 9.0-RC2 -> 9.0-RELEASE upgrade

[Daniel Shahaf]

One of our amd64 servers runs 9.0-RC2 (releng/9.0 at r228325) with a zfs root.

It fails to boot the 9.0-RELEASE (releng/9.0 at r229305) GENERIC kernel
(self compiled) with a mountfrom error:

    http://people.apache.org/~danielsh/infra/loki-20120215-mountfrom.png
    mountfrom> zfs:zroot
    Trying to mount root from zfs:zroot []...
    Mounting from zfs:zroot failed with error 2.

We've tried to upgrade the zpool format 15->28; the symptoms are
unchanged.  (The zroot fs is at version 4.)

Why does 9.0-RC2 boot while 9.0-RELEASE (as /boot/testkernel) doesn't?
What can do to boot 9.0-RELEASE from our zfs root filesystem?

Thanks.

[[[
% uname -a
FreeBSD loki.apache.org 9.0-RC2 FreeBSD 9.0-RC2 #0: Sat Nov 12 18:35:25 UTC 2011     root at farrell.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  amd64
% cat /boot/loader.conf 
# zfs
zfs_load="YES"
vfs.root.mountfrom="zfs:zroot"

# 
ipmi_load="YES"

# Console for DRAC5
boot_multicons="YES"
boot_serial="YES"
comconsole_speed="57600"
console="comconsole,vidconsole"
hint.sio.0.flags=0
hint.sio.1.flags=0x10

accf_http_load="YES"
accf_data_load="YES"
% 

]]]