Debian IPv6 DHCP not working when rebooting

Oh dear... I think you're trying to use the Xen bridging stuff, which is just dire. Set network-script network-dummy in xend-config.sxp, then go with this config:

iface lo
iface lo inet loopback

# Local network, cable labeled M3
auto eth0
allow-hotplug eth0
iface eth0 inet static
    address 192.168.1.184
    netmask 255.255.255.0
    gateway 192.168.1.1

# cable labeled M1
iface eth1 inet manual
    hwaddress ether 00:19:5B:33:86:D5

# cable labeled M2
iface eth2 inet manual
    hwaddress ether 00:19:5B:33:86:D3

auto br-eth1
iface br-eth1 inet manual
  bridge_ports eth1

auto br-eth2
iface br-eth2 inet manual
  bridge_ports eth2

What you then do is tell the domUs to use either the br-eth1 or br-eth2 bridge (as appropriate). Given that you've got cable labelling happening, I'd change the bridge names to something more useful, like perhaps m1 and m2.

I don't have experience with your particular bonding device, but I tried out the following test in a VM on Debian Lenny with a single NIC (eth0). In /etc/network/interfaces:

auto eth0
iface eth0 inet manual
    up /sbin/ifconfig eth0 0.0.0.0

After bringing up eth0, here's what I get from /sbin/ifconfig eth0:

eth0       Link encap:Ethernet  HWaddr 08:00:27:15:8e:d7
           inet6 addr: fe80::a00:27ff:fe15:8ed7/64 Scope:Link
           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
           ...

So I have an IPv6 Link-local address (derived from the MAC address), and no IPv4 address. I am able to ping6 another machine on my local network by its Link-local address, and vice versa, so the interface appears to work.

So, to sum up: Try setting the iface line for your bond0 interface to:

iface bond0 inet manual

and add this line to the end of its configuration stanza:

up /sbin/ifconfig bond0 0.0.0.0

I have no idea whether this is "the right way" to do it, but it works for my simplified case.