freebsd stable - Jul 2013 - 9.1-STABLE zfsloader failure

Yesterday, i updated /usr/src and proceeded to rebuild world + kernel as 
usual on a zfs root install (single disk)

After installing the machine refused to boot with the following error

ZFS: can't find pool by guid

After booting from CD, i first tried to reinstall gptzfsboot, which 
worked but did not solve the problem

I had to revert to the (thankfully existing!) zfsloader.old in order to 
be able to boot

My disk layout is as follows

Geom name: ada0
modified: false
state: OK
fwheads: 16
fwsectors: 63
last: 625142414
first: 34
entries: 128
scheme: GPT
Providers:
1. Name: ada0p1
    Mediasize: 65536 (64k)
    Sectorsize: 512
    Stripesize: 0
    Stripeoffset: 17408
    Mode: r0w0e0
    rawuuid: 400762ff-df42-11e2-95a9-001cb3c56dfb
    rawtype: 83bd6b9d-7f41-11dc-be0b-001560b84f0f
    label: (null)
    length: 65536
    offset: 17408
    type: freebsd-boot
    index: 1
    end: 161
    start: 34
2. Name: ada0p2
    Mediasize: 8589934592 (8.0G)
    Sectorsize: 512
    Stripesize: 0
    Stripeoffset: 86016
    Mode: r1w1e1
    rawuuid: 517ea784-df42-11e2-95a9-001cb3c56dfb
    rawtype: 516e7cb5-6ecf-11d6-8ff8-00022d09712b
    label: swap0
    length: 8589934592
    offset: 86016
    type: freebsd-swap
    index: 2
    end: 16777383
    start: 168
3. Name: ada0p3
    Mediasize: 311482892288 (290G)
    Sectorsize: 512
    Stripesize: 0
    Stripeoffset: 86016
    Mode: r1w1e2
    rawuuid: 5b9f74d2-df42-11e2-95a9-001cb3c56dfb
    rawtype: 516e7cba-6ecf-11d6-8ff8-00022d09712b
    label: disk0
    length: 311482892288
    offset: 8590020608
    type: freebsd-zfs
    index: 3
    end: 625142407
    start: 16777384
Consumers:
1. Name: ada0
    Mediasize: 320072933376 (298G)
    Sectorsize: 512
    Mode: r2w2e5

The new kernel works ok so far

FreeBSD mbp 9.1-STABLE FreeBSD 9.1-STABLE #0 r252531: Wed Jul  3 
00:14:13 CLT 2013     root at mbp:/usr/obj/usr/src/sys/GENERIC  amd64

Is there anything i can do to help debug the problem?

Best regards,
Roberto

I have done some additional research on this,

The machine is a MacBook Pro 4,1, the firmware is only able to boot 
directly to HFS+ formatted GPT partitions. It is able to boot into MBR 
based partitions (which are labeled "Windows" regardless of type)

My disk layout is as follows

# /usr/local/sbin/gdisk /dev/ada0
GPT fdisk (gdisk) version 0.8.5

Partition table scan:
   MBR: protective
   BSD: not present
   APM: not present
   GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): p
Disk /dev/ada0: 625142448 sectors, 298.1 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 33EE1EC7-DF42-11E2-95A9-001CB3C56DFB
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 625142414
Partitions will be aligned on 8-sector boundaries
Total free space is 13 sectors (6.5 KiB)

Number  Start (sector)    End (sector)  Size       Code Name
    1              34             161   64.0 KiB    A501
    2             168        16777383   8.0 GiB     A502 swap0
    3        16777384       625142407   290.1 GiB   A504 disk0

Partition 1 contains the bootloader and is of type freebsd-boot
Partition 2 is of type freebsd-swap
Partition 3 is of type freebsd-zfs and contains the root filesystem

As described, and with the bootloader installed via

gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada0

My system is unable to boot by itself directly. It does boot with the 
help of refit or refind

With 9.1-RELEASE, it used to be possible to create an "Hybrid MBR" 
(http://www.rodsbooks.com/gdisk/hybrid.html) either with refit of gdisk, 
activate the MBR entry that points to the bootloader and have the system 
boot directly without refit or refind

With the new zfsloader from 9.1-STABLE, the system is still able to be 
booted from refit or refind, however, if i create the Hybrid MBR, the 
boot pwouldrocess stops with the message (twice)

ZFS: can't find pool by guid
ZFS: can't find pool by guid

And the install rendered unbootable, either directly or with refit/refind

Mounting the disk from a live CD and replacing /boot/zfsloader with the 
one from 9.1-RELEASE solves the problem (This means i cannot upgrade 
from ZFS v. 28 to the new feature flags spec ?)

I don't know if this behavior qualifies for a problem report ?

Thanks
Roberto