so the solution for me to get all this automajic bridging to work is to add this line to /etc/network/interfaces:
    iface eth0 inet dhcp

Such that my /etc/network/interfaces look like:
  auto lo
  iface lo inet loopback
  auto eth0
  iface eth0 inet dhcp

Then reboot.
That line seems to stop NetworkManager from managing that eth0 interface.
Which then seems to allow the /etc/xen/scripts/network-bridge script to be successful.

Such that after the reboot, this is the output of `sudo ifconfig -a`:

eth0      Link encap:Ethernet  HWaddr 00:22:19:2c:b9:db  
          inet addr:192.168.0.108  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::222:19ff:fe2c:b9db/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:18370 errors:0 dropped:0 overruns:0 frame:0
          TX packets:16217 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:22075797 (22.0 MB)  TX bytes:2582972 (2.5 MB)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:100 errors:0 dropped:0 overruns:0 frame:0
          TX packets:100 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:7600 (7.6 KB)  TX bytes:7600 (7.6 KB)

peth0     Link encap:Ethernet  HWaddr 00:22:19:2c:b9:db  
          inet6 addr: fe80::222:19ff:fe2c:b9db/64 Scope:Link
          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:20195 errors:0 dropped:0 overruns:0 frame:0
          TX packets:16226 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:22774536 (22.7 MB)  TX bytes:2653740 (2.6 MB)
          Interrupt:21 Memory:fe6e0000-fe700000 

I now have a "peth0" interface.
And the output of `sudo brctl show` now shows:

  bridge name	bridge id		STP enabled	interfaces
  eth0		8000.0022192cb9db	no		peth0

Rather than showing the broken "tmpbridge" bridge.

and once I bring up a virtual machine with: `sudo xl create /etc/xen/xen002.cfg`

I see that I have another interface successfully getting created on the dom0 machine:
vif1.0    Link encap:Ethernet  HWaddr fe:ff:ff:ff:ff:ff  
          inet6 addr: fe80::fcff:ffff:feff:ffff/64 Scope:Link
          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:99 errors:0 dropped:0 overruns:0 frame:0
          TX packets:470 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:32 
          RX bytes:10898 (10.8 KB)  TX bytes:88627 (88.6 KB)

NetworkManager still periodically (every 60 seconds) pops up a message of: 
  "Wired network. Disconnected"

so maybe I need to stop the NetworkManager service all together to stop its
pesky meddling with these newly created interfaces or at least have it stop
the irritating, and, I think, non-fatal messages.

I also now have ssh connectivity between my dom0 and domU and vice-versa.
(although, I have to use IP addresses and not the hostnames. Evidently, the DNS
 server does not know about these hosts?)

So I think this "bug" can be closed, although it would be nice if the scripts
took into account Network Manager meddling and either provided a solution
inside the network-bridge script to disable NetworkManager temporarily
or spit out a message to the user about what they need to do.