I've just rebuilt  *.debs  for 4.0.1-rc4 with now problems.
I tested  on the most recent mercurial as well.
root@ServerLDX:/usr/src/xen-4.0-testing.hg# patch -p1 -s -i ../fsimage-zfs-24.patch1
root@ServerLDX:/usr/src/xen-4.0-testing.hg#


Wouldn't it be a fare for you :-

--------------------------------------------------------------------------------------------------------------------
root@ServerLDX:~# dpkg -l '*xen*'|grep "^ii"


ii  libxen4                4.0.1rc4-0ubuntu1     library interface for Xen, a Virtual Machine
ii  libxen4-dev         4.0.1rc4-0ubuntu1     headers for Xen, a Virtual Machine Monitor
ii  linux-image-2.6.32.16-xen    2.6.32.16-xen-10.00.Custom    Linux kernel binary image for version 2.6.32
ii  python-xen-4.0    4.0.1rc4-0ubuntu1     python bindings for Xen, a Virtual Machine M
ii  xen-docs-4.0       4.0.1rc4-0ubuntu1     documentation for XEN, a Virtual Machine Mon
ii  xen-hypervisor-4.0     4.0.1rc4-0ubuntu1   The Xen Hypervisor for i386 and amd64.
ii  xen-utils-4.0         4.0.1rc4-0ubuntu1   XEN administrative tools


root@ServerLDX:~# xm info
host                   : ServerLDX
release                : 2.6.32.16-xen
version                : #1 SMP Sat Jul 10 13:11:11 MSD 2010
machine                : x86_64
nr_cpus                : 4
nr_nodes               : 1
cores_per_socket       : 4
threads_per_core       : 1
cpu_mhz                : 2833
hw_caps                : bfebfbff:20100800:00000000:00000940:0408e3fd:00000000:00000001:00000000
virt_caps              : hvm
total_memory           : 8190
free_memory            : 1671
node_to_cpu            : node0:0-3
node_to_memory         : node0:1671
node_to_dma32_mem      : node0:809
max_node_id            : 0
xen_major              : 4
xen_minor              : 0
xen_extra              : .1-rc4
xen_caps               : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 hvm-3.0-x86_32p hvm-3.0-x86_64
xen_scheduler          : credit
xen_pagesize           : 4096
platform_params        : virt_start=0xffff800000000000
xen_changeset          : unavailable
xen_commandline        :
cc_compiler            : gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)
cc_compile_by          : root
cc_compile_domain      :
cc_compile_date        : Fri Jul 16 18:35:17 MSD 2010
xend_config_format     : 4

I loaded NexentaStor 3.0.2 via pygrub afterwards ( b134 ZFS 24)

root@ServerLDX:~/NexentaStor-Community-3.0.2# xm create -c nexentastor-community-3.0.2-xen.cfg
Using config file "./nexentastor-community-3.0.2-xen.cfg".
Started domain NexentaStor-3.0.2 (id=1)
                                       v4.0.1-rc4 chgset 'unavailable'
SunOS Release 5.11 Version NexentaOS_134d 64-bit
Loading Kernel...
Hostname: myhost
Reading ZFS config: done.
Mounting ZFS filesystems: (6/6)

ZFS Open Storage Appliance (v3.0.2)

myhost console login: root
Password:
Last login: Fri Jul 16 05:58:59 on console
nmc@myhost:/$ show network                                                     
Option ?  interface
==== Interfaces ====
lo0: flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4,VIRTUAL> mtu 8232 index 1
        inet 127.0.0.1 netmask ff000000
xnf0: flags=1000842<BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 3
        inet 0.0.0.0 netmask 0
        ether 0:16:3e:0:0:1
xnf1: flags=1004843<UP,BROADCAST,RUNNING,MULTICAST,DHCP,IPv4> mtu 1500 index 2
        inet 192.168.1.5 netmask ffffff00 broadcast 192.168.1.255
        ether 0:16:3e:0:0:2
lo0: flags=2002000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv6,VIRTUAL> mtu 8252 index 1
        inet6 ::1/128
-------------------------------------------------------------------------------------------------------------------
Here is link 
http://old.nabble.com/-PATCH–RESUBMIT–sync-up-zfs-boot-support-in-pygrub-p28236835.html
to original MRJ's ZFS 24 patch