Thanks Antonie
We are using KVM 2.1.2 as our hypervisor. We typically will using a physical router as the gateway, or a virtual Fortigate. We have tested SilverPeak and like the fact that it can just be set as the default gateway for the VM's as this means we do not need to muck around with extra vlan's.
We also deploy Silverpeak to branches with VRRP between the silverpeak and their branch router, this way if the silverpeak fails the VIP for the default GW will just fail over to the branch router.