I believe we've hit the same issue as others above.

System:
Host - 10.10 ubuntu running the "2.6.35-22-server" kernel
Guest(s) - 9.4 ubuntu running "2.6.32-25-server" kernel

virsh version:
Compiled against library: libvir 0.8.3
Using library: libvir 0.8.3
Using API: QEMU 0.8.3
Running hypervisor: QEMU 0.12.5

NIC in HOST:
hosts have eth0 (public) bridged through br0, eth1 (private) bridged through br1
virbr0 seems to be allocated by KVM on the 192.168.122/24 network
vnet0,1,2...n come up per guest

NICs in GUEST:
plain jane eth0 (public), and eth1 (private).

Symptoms:
From a fresh install of all the above, everything was working flawless... hosts & guests could see both private & public, all were accessible from outside via SSH. IPtables was locked down, and everything was going smooth.

~2 days into server builds, guests cannot be SSH'd into... can't be pinged... VNC'ing into console, we can't ping out. Network is all set up as it was just hours before when all was working, route / ifconfig / arp / and several other commands are run verifying so. Many heads are scratched. Many (including myself) began to cry.

Noticed some chatter implicating AppArmor in  /var/log/messages:
******** SCREEN PASTE START ********
Nov 19 16:38:39 LB-01 kernel: [178667.785748] type=1400 audit(1290213519.522:23): apparmor="STATUS" operation="profile_remove" name="libvirt-cd4fbae6-58be-f3d9-4623-3968f91cf6cb" pid=15435 comm="apparmor_parser"
Nov 19 16:38:47 LB-01 kernel: [178675.423697] type=1400 audit(1290213527.182:24): apparmor="DENIED" operation="open" parent=1231 profile="/usr/lib/libvirt/virt-aa-helper" name="/var/lib/virt/images/baseline-vol" pid=15437 comm="virt-aa-helper" requested_mask="r" denied_mask="r" fsuid=0 ouid=103
Nov 19 16:38:47 LB-01 kernel: [178675.484980] type=1400 audit(1290213527.242:25): apparmor="STATUS" operation="profile_load" name="libvirt-cd4fbae6-58be-f3d9-4623-3968f91cf6cb" pid=15438 comm="apparmor_parser"
Nov 19 16:38:47 LB-01 libvirtd: 16:38:47.981: warning : qemudParsePCIDeviceStrs:1422 : Unexpected exit status '1', qemu probably failed
Nov 19 16:38:47 LB-01 kernel: [178676.214864] device vnet0 entered promiscuous mode
Nov 19 16:38:47 LB-01 kernel: [178676.214886] br0: new device vnet0 does not support netpoll (disabling)
Nov 19 16:38:47 LB-01 kernel: [178676.216191] br0: port 2(vnet0) entering learning state
Nov 19 16:38:47 LB-01 kernel: [178676.216195] br0: port 2(vnet0) entering learning state
Nov 19 16:38:47 LB-01 kernel: [178676.218294] device vnet1 entered promiscuous mode
Nov 19 16:38:47 LB-01 kernel: [178676.218314] br1: new device vnet1 does not support netpoll (disabling)
******** SCREEN PASTE END ********

Crying ceased, Hope shined -- AppArmor disabled, removed, and thrown in a river out back with a concrete block tied to it's ankle.

Rebooted host. Still nothing but VNC into guests. Crying ensued.

Here's what we're still seeing in (seems highly odd -- notice warnings & last line about qemu fail) in /var/log/messages:
******** SCREEN PASTE START ********
Nov 19 18:28:38 LB-01 kernel: [   10.543416] ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
Nov 19 18:28:38 LB-01 kernel: [   10.543557] br0: port 1(eth0) entering learning state
Nov 19 18:28:38 LB-01 kernel: [   10.543560] br0: port 1(eth0) entering learning state
Nov 19 18:28:38 LB-01 kernel: [   10.600665] lo: Disabled Privacy Extensions
Nov 19 18:28:38 LB-01 libvirtd: 18:28:38.408: warning : lxcStartup:1895 : Unable to create cgroup for driver: No such device or address
Nov 19 18:28:38 LB-01 kernel: [   10.809742] e1000e: eth1 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None
Nov 19 18:28:38 LB-01 kernel: [   10.811123] ADDRCONF(NETDEV_CHANGE): eth1: link becomes ready
Nov 19 18:28:38 LB-01 kernel: [   10.811263] br1: port 1(eth1) entering learning state
Nov 19 18:28:38 LB-01 kernel: [   10.811265] br1: port 1(eth1) entering learning state
Nov 19 18:28:46 LB-01 kernel: [   19.209883] br0: port 2(vnet0) entering forwarding state
Nov 19 18:28:46 LB-01 kernel: [   19.209888] br1: port 2(vnet1) entering forwarding state
Nov 19 18:28:47 LB-01 kernel: [   19.529239] br0: port 1(eth0) entering forwarding state
Nov 19 18:28:47 LB-01 kernel: [   19.808654] br1: port 1(eth1) entering forwarding state
Nov 19 19:29:30 LB-01 libvirtd: 19:29:30.221: warning : qemudParsePCIDeviceStrs:1422 : Unexpected exit status '1', qemu probably failed
******** SCREEN PASTE END ********

Hope the above helps get this fixed. The whole stack worked swimmingly until it started blindfolding our guests and making our sysads cry.